Identifying small garden bird ?

Apologies if this is posted in the wrong place...I can never quite get to grips with this forum. Anyway, can someone help with identifying a bird I keep seeing for brief periods of time on my bird table. It is bluet tit size and has a yellow underside...at first I thought it was a blue tit but then it turned round and I notice it has an extraordinarily long and very thin and pointy beak...I mean really long, say 2 inches. My first thought was that this must be some kind of deformity and perhaps it is...I'd go for tree creeper except for the yellowy underneath...Can anyone shed any light atall. Thanks. ! BTW it seems to be coping with life rather well and looks full of energy and healthy...aside from this weird beak phenomenon.
In dog we trust.