Cheers Ian. Someone else has told me exactly the same, about it being a probable escapee. I've looked out for it again today to check out its other leg, but so far I've not seen it.
No such colour ringing projects which I'm aware of for wild birds Chris (not with a single colour ring on one leg); it's perhaps likely to be an escaped avery bird of which I've seen a few in the county, Goldfinch being a favourite. Not being able to confirm a metal ring on the other leg is a bit of a hinderance to submitting it to the BTO anyway though.
Today I noticed a goldfinch in the garden with a single yellow ring on its left leg (the right leg was obscured). Is there any way I can find out when and where this bird was ringed?