You'd have to be a bit crazy to pay 50 quid per tyre for fitting. I always supply my own tyres (3-4 sets per year, around £900 per set for Bridgestone RF's) and the most expensive local fitter charges 20 quid per tyre to fit. Someone I know took his to Kwikfit who charged 5 quid per tyre, ha bargain!
Usually takes about an hour and a half for 4 tyres.
Nissan quoted me £2600 for a set of Dunlops earlier this year, their profit in that would have been around £2000. **** that!