Sound like the compressor clutch has shat itself, can be done on the car, not that easy but doable.
They will also replace the receiver/dryer ( fancy name for a filter ) required by law, and o rings, evacuate and replenish oil and recharge system. Thats the proper way to do it.
Most workshops these days get someone in or outsource it to a different workshop, so save yourself a drive and find a local auto electrician that specialising in a/c, tip, look for one that does trucks, their as tight as, so if they can keep them happy the a/c people will be good.
Thanks Phil


Terry