We got a good supplier through these people
http://www.discountedheating.co.uk They have been around for a few years
now, and have some good deals. Worth a try, eh.

Currently looking for radiators too and I can confirm, this outfit appears
to be cheapest I've found for the wider sizes. Places like Screwfix and B&Q
do offer much cheaper radiators at smaller sizes but as the original poster
found out, they don't do the big ones.

Cheers, Rob.