Who flew the aircraft back? Did the Instructor take control? What was the defect?
I'm probably out of tune with most here, but when it developed a fault, you were no longer able to conduct the exercise you were expecting and paying for. It might be fair to charge you for the first 20 mins and a landing, but I think the return leg wasn't training, it was a "forced" return to base over which you had no control or decision and it's their problem.
If the defect is something that could/should have reasonably been picked up on the "A" check, then you shouldn't pay at all.