On 20 September Bob started a new trick. Occasionally, in a left turn, the gear seemed to let go. He had not done it when he first arrived.
The box is a Borg-Warner 35. Having done some research, I found that the pump pickup sits on the left-hand side. Turn left and whatever fluid is in there sloshes the other way. If the level is low, or the filter is blocked, the pump drinks air instead of ATF, and the drive goes soft for a moment.
Fingers crossed it is low on fluid, and not the filter. Dropping the pan is a much bigger, messier job than a simple top-up.
The identification plate on the car says it should have been a Borg-Warner 65. The plate on the gearbox says BW35. The paperwork that came with him includes a gearbox recondition by VMTP Midlands Limited in November 2018, at 94,666 miles, costing £1,530.50. Whether they swapped the box, or it had always been a BW35 and the vehicle plate was wrong, who knows.