What is this bird? Any ideas?

Not particularly a bird watcher but spotted this little critter in my garden (South-East UK) & can't find it in any book. It was sparrow size & ast first, I thought it was a goldfinch. Same kind of build as a finch & was next to a sparrow so could compare size - which was the same. It had a BRIGHT red head - just the top, like a little cap. It's tummy plummage was bright yellow & the rest was black, white & brown. Anyone shed any light? Sensible answers appreciated. Thanks