He didn't actually say he was getting the arches smoothed into the back bumper, just that he wanted the arches smoothed in
Even to just get the back arches blended into the bumper, it'd take alot of fibreglassing and filler to blend it in and make it look good so would be alot more than £300 IMO.
If you aren't wanting to blend the arches into the car it means that the blending and painting would need to be done on the car and would be a real ball-ache, so you'll probably get quoted a ridiculous price just to put you off lol.
I really wouldn't advise blenind the bumper into the arches if that's what you're thinking of, because any flex in that bumper is just going to cause the filler/fibreglass to crack. Just take it back to the bodyshop and tell them to fit it right