I now have it repaired. The procedure for anyone else:
The gearwheel needs to be removed with some sort of puller. It is not too tight so some flat aluminium behind the gear will be plenty. Some taps with a hammer and a block of wood would also remove the gear provided the gear was supported. A puller is the best option. The large circlip can then be removed. The large circlip is close to impossible to remove until the gear wheel is removed. Circlip pliers with an unusual bend might do it. The seal is 10mm x 35mm x W7mm. This is a standard size seal but may not be commonly stocked. Reassembly is obvious. There are design issues here: The shaft runs in a single bushing in the crankcase and is perhaps encouraged to float by the seal. The bushing has no entry or exit for splashed oil. Thus, purge of wear debris is perhaps prevented. I wondered about drilling a small hole on the top to allow oil to run into the bush. It is difficult to test the state of wear of the bush. Also, when oil is put into the bush for reassembly, it creates an airlock as the shaft acts as a piston which impedes assembly. It is difficult to detect whether excessive force is being used during mounting of the pump. Another issue is that the seal runs directly on the shaft. Thus a slight groove may occur in the shaft.
I initially tried Evans waterless coolant. I reverted to green coolant. I had two concerns:
If coolant gets into the gear oil, we detect a change in gear oil colour. I'm not sure if this is the case with waterless coolant. Secondly, if the engine gets too hot she boils and we use the boiling as a measure of: "She got a bit hot." If the waterless coolant does not boil, we cannot tell if the engine is overheating as we have no temperature monitoring in place.
There appears to be no provision or space to add a second seal and run excess water to the atmosphere. From memory, I did this on some early Shercos. I fitted two seals and Araldited some very small bore aluminium tube from the model shop to vent escaping water to the outside world where it could be seen. (On the Ossa, I have the radiator vent pipe slightly outside so that I can see any water dribbles.)
The main problem is the large circlip hidden behind the gear wheel. Seals normally retain their position with their interference fit. I can't see that pressure build-up would be enough to move the seal. Mine now runs without the circlip. I have previously used Tectaloy concentrate and did not bother to add water. But some water may be useful to allow it to boil when too hot. I never really thought about it before.
Hope that helps. Andy Chalkley