The Arthurian legends are shrouded in much mystery, and tales of the end of King Arthur's reign are no less varied and colourful than his earlier adventures. However, this much we do know: having been mortally wounded in battle with Mordred at Camlann, Arthur was taken to the Isle of Avalon. Such were the healing powers of this mystical isle that Arthur was saved, but it was his destiny to rest there until Britain might need him once more.

Many centuries passed, and many troubles befell the people of Britain, threatening to stir King Arthur from his rest. However, Britain always managed to save itself before Arthur's intervention became necessary; until, that is, the return of Morgana ...

Some legends tell of Morgana's defeat and her death at the hand of Merlin, by the blade of Excalibur. That she was struck down is indeed true; but the powers of Dark Magic run deep, and Morgana's mastery of those powers was second to none. Like Arthur, she too lay in rest, tended to over the centuries by her most loyal followers and their descendants. Unlike Arthur, however, she was not waiting for the hour of Britain's greatest need. No, she was waiting for word of the artefact that could fully restore her power and with which she would become truly invincible: The Holy Grail.

When Morgana's minions finally learned of the Holy Grail's location at Bedfont Lakes, they began their incantations to rouse her; immortal power is within her grasp at last. Fortunately the resonance of the incantations was sufficient to alert those loyal to Arthur to do the same. But who will win in the race to find the Holy Grail?

To find this cache you will need a smartphone with a GPS receiver, such as an Android, an iPhone or a Windows phone. The cache is played on a web page. The phone must have a web browser that supports "location services" using the GPSr - most do.

The cache is not a puzzle, but you are strongly advised to read all the cache page before you leave home. If you don't read it all, at least read and remember the red bits :-)

Finding the cache involves a walk of about 3 miles around Bedfont Lakes Country Park, and it should take a couple of hours.

The park is open from 08:00 to dusk, or to 21:00 if that is earlier.

There are entrances to the west at Clockhouse Lane and to the east at Bedfont Road, with car parks at both. The listed co-ordinates are for Clockhouse Lane, but as far as the cache is concerned there is little to choose between them.

Bus H26 runs between Feltham, Sparrow Farm and Hatton Cross Underground station, passing the park. H25 runs from Hanworth to Hatton Cross via Bedfont Green, from where you can walk to the park. Route 116 runs from Hounslow Bus Station to Ashford Hospital via Bedfont Green. Routes 555 and 557 visit Ashford, which is close enough, but I recommend not walking from that direction - there is a narrow bridge over the railway, and it has no footpath.

There are public conveniences near the Clockhouse Lane entrance.

On Saturday mornings at 09:00 there is an organised 5Km run in the park. Best to avoid this time as the park is busy and the car park may be full. It's pretty much over and done by 10:00.

Dogs are permitted in much of the park, but not in the nature reserve area. The route passes through this area, so if you have a dog with you it will be necessary for somebody to be available who does not mind staying outside with it.

All terrain buggys should have no problem getting round, and normal buggies should be OK during dry spells, but see the "limited mobility" section below for more detail.

Cycling has a rather strange status - it is supposedly both forbidden under the bylaws yet permitted in much of the park provided care is taken and pedestrians are given priority. However, they are expressly forbidden in the nature reserve area through which you need to pass. I would recommend that you don't use a cycle - quite apart from the nature reserve, there are some locations and activities where it would be a hindrance.

Setting up your phone.

The cache uses your mobile browser to display a web page. Browsers vary in the quality of their support for location services, but all offer less control and reliability than would an app. Some phones update every second, some update every second only when they have a fix, and some update only when you move. This makes it impossible for me to differentiate between a lost signal, a browser that has stopped updating, and a player standing still.

I get more reliable operation on my iPhone by starting up an app that uses the GPSr, (e.g. the Groundspeak app) and leaving it running in the background. If the web page stops updating (shows "No fix"), briefly revisiting the app kicks it off again.

Phones that have completed the circuit OK:

• iPhone 4S + Safari
• HTC Desire HD + Android 2.3.3 + Maxthon
• Galaxy Note 2 + Chrome
• Galaxy S2 + stock browser (gave "No fix" when the player stopped)
• Galaxy S3
• HTC Wildfire S

You can use this test page to see if your smartphone and browser will provide sufficient accuracy. If your phone asks to use location services when you visit the page, you need to allow it to do so. The accuracy reported should be no greater than 25, and preferably less than 15. You may have to wait for a minute to get a fix, after which the accuracy should gradually improve.

You may find it useful temporarily to increase the screen timeout - when the display goes blank, it's likely the GPSr stops working. On iPhone it is in Settings / General / Auto-Lock. On Android it is Settings / Display / Screen Timeout. Having said that, if it has gone off, on my iPhone it recovers very quickly when I turn it on again.

It's a good idea to ensure your phone is fully charged before you start, as running a phone with the display and the GPSr permanently enabled is hard on the batteries.

Playing the game.

I said it before, but it's worth repeating - it is advisable to read all of the cache page before you leave, and it is a very good idea to print out the "game map" above. This has features of the park "interpreted" to fit the game (though you must use LOTs of imagination). At points in the game you will be told to go to a location, and the printed map gives a useful overview of the whole playing area.

The phone screen displays the same map, but it is always shown "north up". The printed map will be especially useful if you are used to looking at a map in the direction you are facing.

Start playing by visiting one of the two web pages below, though you won't be able to do very much unless you are already at Bedfont Lakes.

While the game play is identical, the two pages provide a different user interface. Here is a summary of the differences:

On an iPhone, and probably the others too, you can have both views open at the same time and very easily switch between them.

There are two major display modes.

Most of the time you will see a screen map. Below the map is a description of your task for the current stage. Read all of it, carefully. You may have to scroll down to see it all.

When you get close to a waypoint, the map is replaced by a question and answer section where you enter your responses for that stage. If you are using kar.htm, and your map is zoomed a long way in and dragged to the right, this may be "out of view" to the left. Zoom out and drag the view to see them.

When you are being prompted for an answer, there will be a 'Send' button at the bottom of the page. If you can't see it, drag it into view.

Beside the 'Send' button is a 'Hint' button. This gives fairly explicit help in case you get stuck, but it would be good to see how many stages you can solve without resorting to it.

If you move away from the waypoint the display will revert to the map - to enter the answers just go back a little towards the waypoint.

The stage you have reached is stored on your phone, so you can come back another day and resume where you left off. If for any reason you wish to start again at the beginning, visit http://www.amberel.com/ka/kas.htm and press 'Restart'.

Start your quest at Millennium Hill.

Finally, you will get more out of the game if you give your imagination free rein.

For those with limited mobility.

Most of the route is on quite hard packed gritty paths, though even these have suffered over this wet winter. I think probably these would be suitable for a wheelchair, though the grit might be uncomfortable if it got on the hands. But there are a couple of places where the route climbs small but steep hills on a grassy and not very even surface. I think this would be a very serious challenge for a wheelchair, especially when it is wet and muddy. There are alternative paths available to the wheelchair user if they are accompanying a group, but it would mean missing some of the stages which are necessarily on the hills in order to fit the features of the park into the story line.

The "Nature Reserve" section gets very muddy - not deep mud, but very slippery. This section is also gated. It may be advisable to ask the rangers at the Clockhouse Lane entrance about access to this area prior to starting.

At the "Motte" area there is ramp access in addition to the steps, but you need to go further round to the north-east.

There is one activity that requires some rapid movement on uncut (but not unduly long) grass, but this area doesn't drain well, so can get wet and very soft.

If you were able to get up the hills and into the Nature Reserve, nearly all the waypoints and answers would be visible from a wheelchair without getting out of it. But for the first one (at the top of a steep grassy hill) the "question to answer" involves looking on top of a stone pillar a metre high.

My view, as a non-expert in these matters, is that a wheelchair user would not be able to complete the cache by themselves, but could accompany a group for a substantial part of the route.

Many thanks to Hounslow Council for permission to place the cache here, and to the rangers of Bedfont Lakes for their friendly co-operation.

I am very grateful to abanazar for his immense help with the story line and text, and to UrsusBear for some ideas on game play.