Valve clearance service (including oil change, filter checks/changes, plugs etc), assuming no valves need changing, will cost around £350 at a dealer and around £200-£250 at an independent, it's a 3.5 to 4 hour job, chances are the dealer will charge you the 4 hour book price and the independent will charge you the 3.5 hours it actually takes him.
If any valves need adjusting then it will cost more, not just for the parts, but the time it takes to replace the shims.