The job took me rather longer than morris1000's impressive 5 hours. I've no idea how he managed to get the box back in without removing either the engine or the gearbox crossmember. Getting the top of the bell housing past the steering rack was very awkward even with the rear of the engine lowered as far as it would go and with a pit below the car so I could can stand up rather than lying flat on my back.
The rebuilt gearbox had one slight fault - the spring-loaded detent that stops you accidentally changing into reverse instead of fourth was jammed. Upon closer inspection it appeared that some water had got into the the back of the remote control unit (there was no gearstick fitted, so it had an open hole on the top) and it had rusted slightly inside. The simplest solution was to just swap the remote control unit from my old box (which was still in excellent condition) onto the new one.
I did have to cut a corner to get the car back on the road this weekend. The clutch pedal doesn't return fully while the brake is pressed (the brake pedal hinges on the clutch shaft). I was hoping I would be able to get away with just replacing the brake pedal bushes, but upon taking the assembly apart (rather an awkward task in itself as the spacer tube had rusted onto the shaft) I discovered the clutch shaft to be quite badly worn and pitted from rusting. Several other parts of the clutch linkage are badly worn too. If I'd waited until I could get hold of the parts to rebuild it all properly it would have meant several more days off the road, so I decided to just clean and grease the old parts and reassemble them.
Here is a pit-eye view of the underside of the car after replacing the gearbox.