I had a look at the 'Fixed Price Servicing' page on the VW UK website and was impressed with the pricing. The offer an MOT for £30, or free if you're on the £299 3 year servicing deal.
My car needs and MOT, so I phone my preferred dealer (Martins of Basingstoke) to book it. I asked how much it would be and was told £54.50. I quoted the VW UK website price and was put on hold whilst the lady "checked with her manager".
She came back to me and said "Those prices are only for participating retailers. We're not a participating retailer. However, we will do the MOT for £30".
I don't recall anything about "at participating retailers" on the website!