you could always use one of those little rollers on a stick... should be able to get down about a foot behind it ?

Get one of those little rollers for painting behind radiators
http://www.diy.com/departments/bq-4-radiator-roller-set/189764_BQ.prd

So THATS what those are for

Cheers guys

There are brushes available too but a rad roller is probably the best option, some £1 shops used to have them. Maybe still do. Normally people only see the first few inches down behind a radiator.

If you have some old wall paper or packing paper you don't want fold that over the radiator first, it will stop paint getting on that.

Thanks, I thought about doing something like that too. I should be attempting it next weekend I reckon, all things being well. Gf's parents have a big dust sheet I can borrow, I just need the other bits.