See here:
https://www.thegreenage.co.uk/do-radiator-reflectors-work/
(Note that the figures quoted for savings there are those which would have applied 8 years ago. You'll obviously save more now but I'm sure that the price of the foil will have gone up too!)
Thanks bhg, I do look for cheaper ways before spending but in the case it wouldnt be worth the hassle compared to the outlay, plus the chanes are it would look very messy homemade.
A couple of observations after reading the first link:-
1 Radiators do radiate heat back to the wall, but their main method of providing heat to a room is by convection. They should rally be called convectors.
2 Crinkly foil is just as good as smooth foil, according to the HVCA data I used for heat-loss calculations in the seventies. In fact, crinkly foil was better than smooth.
Kitchen foil is so cheap that it must be a good option for saving a bit of energy.
It's worth mentioning that the foil should be against the wall, not attached to the radiator. The idea is to let the heat out of thr radiator but prevent it from going through the wall. It follows that outside walls gain most from this; with inside walls the heat will go into the nest room which you might not want to be as warm.