I saw them just this last Thursday in Ipswich and this was one of the highlights of the evening, done with great affection in the style of 'Take Five' Dave Brubeck. Of course, this was particularly poignant this year as Brubeck's sad demise was announced just earlier this month.
Unfortunately, this track isn't available on YouTube as I heard it and much of the St Agnes stuff to be had on there is poorly recorded live material but if you're interested enough to seek them out - and I'd suggest you do - you can hear it on Spotify.
A word of warning though; 'God Rest Ye' is one of a kind of 'Best of' compilation available on Spotify but I'd suggest St Agnes Fountain are very much a live experience and I wouldn't want you distracted by the recorded output that pretty much fails to convey the great spirit you get from watching them live. It's probably too late to catch them this year but I'd put it in your diary for next.
Update 2015: Despite reservations, there's an interesting alternative version to the jazz inspired 'Merry Gentleman' included below where the Aggies all play one guitar. It at least gives a feel to the diversity of the band.