I think most shops charge between £150 and £250 to do it, including paint.
Unless you have it all already, hire of a welder = £50 a day I guess, Paint/laquer/hardner/primer/wet and dry/sandpaper = £150 and hire of a spray gun and compressor = £50 per day
Total = £250 same as getting a body shop to do it in the first place.
*EDIT* Forgot to include the price of a piece of steel in that (not sure how much steel goes for at the minute but at a guess it'd be more than a couple of quid, unless your gonna use an old car panel)