Martin wrote:
Maybe it was just a sparrow. :-)

or a dimmock/dunnock
http://www.garden-birds.co.uk/birds/dunnock.htm

Ah, could well be. Well called. Or, browsing the site, perhaps
a siskin? They have the shape and the yellower tone and the
stumpier beak, but don't seem to have the markings.

Good site, though.