I'm not sure, but the way I read it, had you left it there for all the faults to be dealt with, the 10 day limit would not apply. As you took it away to fix some of the issues, then the 10 day limit applies.
MOT stations can charge what they like, within the maximum, so worth trying to negotiate a lower fee, but always best to do a pre-MOT, to pick up obvious, easily fixed issues before presenting it for an MOT.