I have now had a look at my records and back in 2016 I noticed that this O/D was slow to drop out of O/D when it was hot and I had to ask a local classic friendly garage to change the solenoid valve over to one that I had taken off another O/D unit as I could not undo the one on the car. When this was done the problem went away.
I have also found that the workshop manual provides a list of faults that could cause the problem and the actions to be taken. Some are a bit more than I would wish to do with the car on axle stands so today I removed the solenoid and checked it out. As the workshop manual did not indicate that the gearbox needed to be drained, I decided to give it try without draining the gearbox. I just had a container in place to catch any oil that came out. When I removed the solenoid, a trickle of oil did come out but if I had a replacement to hand the loss would have been very little. When I applied 12volts to the solenoid the plunger seemed to be sticking in the engaged position so I soaked the piston section in white spirit but this did not cure the problem. I noticed that there was a very small circlip that kept the piston arrangement in so I struggled to take this circlip out. The next struggle was to get the plunger out but in the end I achieved this. The workshop manuals and the parts catalogue do not go down to this level so I have not been able to replace the O rings on the plunger. Having given all the bits a good clean I put it back together and retested it across the battery. It now seemed to be working fine so I refitted it and topped up the unit. It needed about 0.5 Ltr so it was a pity that I did not drain the lot before removing the solenoid.
See photo of stripped plunger arrangement.
As the car is on axle stands I ran it up in third and engaged O/D it seemed to engage and disengage normally. At 30 mph the revs reading was about 2200 and when O/D was engaged the revs dropped to about 1900. These figures are what I use when the car is out on a run and I wish to check if O/D is engaged or not.
As it was now about 16:00 hours I decided that I would leave the true road test until tomorrow. Lets hope that it does not stick again when it warms up.