Hi Kate,
I find MoreVeg good, a good range of all vegetables and particularly good for small quantities at low prices, handy for trying out lots of different varieties:
Nicky’s Nursery also has a good range and has started to sell a limited range in smaller and cheaper packets:
http://www.nickys-nursery.co.uk
If you have Charles’ books handy, I think he lists some seed suppliers that he uses towards the back, maybe in the references section.
Darren