I've seen a great deal of these young Blue tits around recently. To start with I had to check that were in fact Blue tits, since they're mostly grey and yellow, without the classic blue and white on the head. The two pictures on the RSPB Blue tit page (click the "1" and "2" links to flick between them) show the official differences quite clearly. Below is a picture of my own of an adult to compare. The juveniles also seem smaller and thinner than the adults, but maybe that's just because it's hot and they're not plumped up.
My willow tree is absolutely full of these little critters and this morning they woke me up with sounds of nibbling on the window ledge, there must be some delicacy lodged in the wood. It would seem there are more of the grey variety about at present, it’s a rare treat to see a really true blue tit.