Hi Rupert, and welcome to the forums.
A perplexing one, that's for sure.
Either the recent work is entirely unrelated and its pure coincidence (which can happen), or its something caused by the garage (which is where I think you believe the responsibility lies
I'm fairly sure your car will have a cable-operated throttle (rather than drive by wire), so I'd be inclined to trace the throttle cable from the pedal box, through the bulkhead to the engine, and look for signs of fraying, dirt, bends or twists which would impede its operation.
Also, if the cable is sited (or has been moved) close to a heat source (i.e. engine or exhaust manifold), then this can cause the cable to 'stick' within the sheath.
I'd also make my disatisfaction known to the garage (politely, but firmly).
Hope this has helped,