I really cannot see why the garage should have any reason to touch the machinations of the auto box as it does not play any part in the m.o.t. I would suggest that either the gear box oil (TQF) needs topping up or your torque converter is up the swany. from what you describe the transmission fluid(TQF) needs topping up as your vehicle behaves out of character when the TQF loses viscosity (transmission fluid gets thinner as the car warms up) What I would suggest is there are two level marks on the dipstick one for hot reading and the other for cold I would suggest that these have been confused if looked at at all the lower mark is for cold reading and the higher one for hot either way I bet there is not enough in it.