Thank you for all the input guys.
Unfortunately there has been very little support for older electric cars in the UK, even less in the backwater where I live. I am a 66 y.o. woman. I do not undertake these tasks for pleasure. I do it because I believe in electric cars, very much enjoy using mine and want it to be reliable.
The NHTSA notice is for the USA. I live in England, part of the United Kingdom. There has been no recall I am aware of for this (I have checked online) as 2G is still active here. (It may be discontinued in 2025.) When I purchased the car I was unable to access Carwings. The dealer was no help so I have never had access. Similarly they have not told me Carwings was no longer operational (at the time I never thought to ask just accepting what I have read on the forums). After previous experience with them I would not trust them with any work. Certainly not if I want the car back the same day and charged!
I believe my problems were caused partly by Carwings being deactivated but mainly due to a very poor 2G mobile signal. With my 2G phone I had to climb onto the roof to get a useable signal. Completely disconnecting the TCU guarantees that it will never again be a drain.
12v battery life has been 12 to 20 months before there were problems, which seems to be common on many makes of electric car. I did not think to use a clamp meter sooner as I mistakenly believed from reading on this and other forums that all my problems with the lead acid battery were due to the vagaries of the charging system causing sulphation by the low float charge. It was only after the car flattened a new but cheap (Mr. Snappy 45Ah) battery in 36 hours that I resumed in earnest my search of the online forums. I will buy or borrow a clamp meter presently and check what is going on. Rather annoyingly the morning I removed the TCU the Mr. Snappy battery was still at 12.2 volts after 48 hours. I am keeping a close watch on the Optima 38Ah battery... And trusting that it lives up to it's makers claims. If it does then I have saved £50 on the cost of the 48Ah battery. If it fails I shall go away and mutter rude things in a quiet place!
Overnight the temperature dropped from 14 to 0 centigrade but the voltage remained at 12.62 (ignition off) and it was still there at lunch time today after driving 45 miles.
Regards to all, Emma