When regarding cambelt i feel easier paying that little more to a dealer just incase somethings goes wrong.
I can only speak about my local dealer 'White Rose VW' in crosshills, they have always reduced the rates, gone out of there way to fit me in even when booked up for a few weeks. Example of this came on my mk4 when the aux belt tensioner failed on a saturday morning. Gave them a ring asking cost ect and when it could be done, reply.....' Drop it in now and i will fit it in and it will be ready for 1pm when we finish'
They have fought my corner on many an out of warranty claim before, even when the window regs failed on the mk4 i paid about £90 for it to be fixed, day after they rang me, refunded the money and said we've done it good will and will chase VW HQ for the money, also can we have it back as we will do the otherside for you also!!.
It always pays to build up a good rapport with your dealer and ask for a price reduction!