The posted co-ordinates are for the Baptist Free Church of Barton Mills, the first of two churches you will be visiting to find this cache.
In the churchyard you will find a memorial to the daughter of Captain Will Taylor of Whitby, and another to the widow of the late T Ellington. Take each two digit number you see on the appropriate faces of these memorials and add them together. The sum is the last three digits of your northings.
Now make your way to the church of St Mary, where you will find a cottage presented to the church. Within the churchyard you will find some water provided by many friends. Again, using only the two digit numbers you see, take the arabic numbers from each memorial and add them together. Now take away the two non-arabic two digit numbers (one roman and one a word) from your sum, and you will have the last three digits of your eastings.
Of course, as per the title of the cache, it's in the ivy - because everyone loves a micro in the ivy! (No? Neither do I - but at least it's not as small as a film canister...)
This cache has been designed to be found along a circular route from and to Mildenhall. By the time you reach this cache along this route, you will hopefully have already found Something Wacky (Fishy Willow?). GS caches along the route are Down by the river 3, 1 and 2.