Geoff,
A company like Dentmaster wouldn't be likely to charge as much as that for simple dent removal. The answer may lie close to home

I'd guess about the £100 mark for a professional alloy refurb, but it could be difficult to get a perfect colour and lacquer finish with the other wheels, so unless there is a reputable refurbisher in your area, I would stick with the dealer to get this done. If it isn't perfect, you can always take it back and complain, may even get yourself a replacement if they can't do it effectively !
Standard practice these days for dealers to insist on their own bodyshops to ensure the job is done in line with manufacturers warranty specifications. The plus side is that the warranty is still valid and you can take it back again if the repair fails within the warranty period. Given the age of the car, I would go this route. Are you aware of any good/adverse reports on the quality of their repair work - different dealers do have different reputations.
Good luck, chill and enjoy

Cass