Harvey,
The black machine is currently with the local agent to have a driveshaft replaced. Whilst I was there I talked to them about 'solenoid C' etc. and it was as if I was speaking Klingon. They clearly weren't keen on taking the gearbox apart and were referring me to a gearbox specialist that they have used in the past. Given that the gearbox is working fine apart from the locking up, it seems daft to have it rebuilt, so I am keen to try out your idea as it does make a lot of sense. I can see that the biggest problems are going to be to explain what needs to be done to the gearbox specialist and to find the part numbers for the replacement valve and clutch plates. I have seen the page from the workshop manual that relates to this area in your locker, but it does seem to lack detail. Is there any more information available about what needs to be done? Secondly, do you have any idea of the part numbers for he replacement bits that I will need?
Thanks very much for your support so far.