The actual fault, thermostat and heat control component failure issues, i believe, can bring in ecu limp mode along with the fault warning indicator.
when you picked the car up, you said it "No Power" which sounds like limp mode to me, so i wonder if, following the physical repair itself, (assuming this has been effectively diagnosed and ALL relevant components replaced properly in the first place!) it could be that the "muppets" (ha ha!!) have not cleared faults in the ecu.
and not just the inital fault code, but the other codes that will have been picked up from unplugging, disconnecting, and dismantling components to effect the thermostat replacement, its likely these that have brought on limp mode.
as it is an engine heat control issue, the ecu will retain the fault code, error warning, and limp mode until otherwise instructed too, it won't reset itself just because it has a new thermostat.
Yes the garage should be repairing all this, yes they should have done it properly in the first place, and yes you should report them up to the underwriters, but not before you are 100% satisfied that your car is fault code free, test driven for a day, and remains fault code free by plugging in again to sign the job off as completed to your satsifaction.
don't sign anything accepting the car, which i would have thought they would have asked you to do, so that they can claim from the underwriters, they haven;t signed on your behalf have they seeing as you picked up after hours?