Depends where you are really. A 2006 bike wont really be worth any more with the dealer stamps than without now and the money you spend on getting the stamps will definitely be more than any additional value vs. doing it yourself. If you do it yourself, just write in the book and keep the receipts for oil etc. A bundle of receipts to me is worth more than a dealer stamp as it shows what you have done rather than got the dealer to wipe an oily rag round the engine.
If you need a dealer to do it for warranty reasons or your not confident enough to do it yourself, any dealer will do, just tell them to use Yamaha bits if its under warranty. I would recommend T Northeast (Horley) if you are in the Gatwick area but not P&H (Crawley).