In reply to James Malloch:
It is to do with the level of service you get.
A council MOT station does only that, MOT's. They do not repair your vehicle.
Many MOT stations have full garages attached, and some of the more unscrupulous places will always "find" something wrong, which of course they can fix and charge you for. Using the council garages is a method of ensuring an honest MOT.
Really depends on how good you are with cars. Myself, I happen to be an engineer, so will happily argue the toss with an MOT garage, as I know what I am talking about. Those who are not as mechanically minded are a bit more vulnerable.