I was sure that Screwfix did rads longer than 1200 but other than the
300mm ones which can be got up to 1400 I can only see 1200 as being
the longest .
Can anyone suggest an alternative source for rads with a comparable
price?

Been there, done that and the answer is "No"... as another posted
http://www.discountedheating.co.uk is the best I've found and they are ~20%
more expensive than ScrewFix & B&Q. But these guys shift big bulk and
probably get excellent discounts.

eBay is even worse!!

Cheers, Rob.