On assumption the your mechanic is allowing for a head skim, and all other assiciated parts to do the job properly, I reckon £800 is a fair price. Will depend on bent valves, pushrods, etc. If he is pricing it up with genuine parts, wont take long to add up.
With valves, pushrods, skim, gaskets, filters, coolant, possible water pump, £1300 would be quite realistic allowing for a sensible hourly rate.
Be thankful you don't live in Aberdeen, most dealer hourly rates are £90+ per hour!!