in my opinion a local independant garage or specialist will do a better job as, more parts are changed normally as you can specify and it would be cheaper. nothing stoping you buying the bits you want changing from seat and doing it that way just pay the labour or do it yourself.
Most services are a oil and filter + inspection. They charge around £ 200 at dealers, to me thats just rip off and in all honesty paying for the stamp.
Every year i would prefer to change oil oil filter pollen and air, and maybe plugs depending. For the money it costs for all those bits its worth it an extra 10 quid for a filter isnt too much to spend every service to make sure you have a clean filter, pollen etc. all these bits with genuine oil dont come to over £60.
Also the cambelt change is overpriced, its basically like doing an 8v car only 1 pulley and its not the hardest thing ive done a few in around an hour. But the dealers have more overheads to pay IMO. maybe this is why the price is so high.
I would have to disagree that an independent/specialist garage would do a better job, they could do an equal job agreed, a dealer will happily change parts you request and being cheaper isn't a qualifier for a better job.
Taking it to a dealers you are obviously paying to get the stamp in the book, the idea being that the stamp indicates the vehicle has been serviced to the manufacturers criteria with the specified parts and consumables. As you rightly point out dealers are so expensive due to the overheads and also the staff due to the number of departments that make up a dealership, unfortunately most of which are admin/managerial. Add on to that the cost of special tools and diagnostic equipment that dealerships must have, staff training.
Take it to an independant garage and you usually deal with the mechanics or the boss so there's less bodies to pay in between.
As for doing the servicing yourself, yes you don't get a stamp but keep the receipts for the parts as a little proof that they have been changed, once a car gets past a certain age service history is relied on less anyway.
Personally I will be servicing my Leon myself because theres no point paying somebody to do it for me.