Does an MOT retest have to be done at the same garage as its original MOT?
Yes or they have to do a full re test not just check what it failed on.
And remember not all fail items qualify for a free retest.
If it’s your Volvo FH You’ve asked about the Steering then No - You can take elsewhere.
A free retest is often some kind of special deal related to the individual garage. The idea is that you pay for any repairs that are needed there and then, rather than shop around for somewhere else cheaper, and then pay for a full retest at a third party establishment. They’d have to do a full test from scratch, because the data from the first test isn’t collated between the two rival garages. Obviously, no one is going to be doing a full test for free, unless it relates to some other kind of third-party deal, for example I got a free MOT at a halfords garage once because I’d taken out a Tescos car insurance policy…