It's a male Sparrowhawk, Gillian. They are specialist predators of small birds and are quite common in gardens especially if you feed birds. Shame about the Goldfinch but Sparrowhawks have to eat too.
It is indeed a male sparrowhawk. Merlins do make their way down to the lowlands in the winter but are birds of big open spaces rather than back gardens. The absolute clincher ID tip is the big orange eyes. Merlins, like all falcons found in the UK (and my avatar bird) have dark irises.
Thank you very much for your reply Tony. It is surprising what birds come into your garden when you start feeding them. Yes poor goldfinches, but there are plenty more.
Lovely photo Gillian. I have a rather large female Sparrowhawk visiting my garden. She nearly took my head off the other day chasing a bird and she's very partial to my poor Collared Doves.