Marsh Tit
Poecile palustris (ex. Parus palustris)

Marsh Tits are moderately uncommon in Britain and are superficially similar to the related Willow Tit. Thay can be sometimes seen at bird tables and aren't too shy. The easiest way to tell them apart from Willow Tits is the call - a startling 'pi-chay'.
 

1: Chard Reservoir, Somerset, 21/01/2007.