Just to close the circle on this thread, I decided to change the gearbox now as it had suddenly become very noisy, and so I approached TD Fitchett about a service exchange box. They didn't have this particular item in stock, but said they could rebuild mine in 2/3 days. The box was removed last Thursday and sent via Parcelforce that day, and was received by Fitchett's on Friday morning. I got a telephone call from Tony Fitchett at 2.30 to say he had completed the rebuild and he would return the item the following day - WOW! We received the rebuilt box yesterday afternoon.
TD Fitchett may not have the fancy websites of some other suppliers, but their service can only be described as exemplary.
I have no connection with TD Fitchett except as a very satisfied customer.
Sam