So a lot more investigation later...
Our local Nissan dealer knows nothing and have only done recall work on the few Leafs (Leaves?) in the area. They replaced one traction battery under warranty and due to lack of facilities hired a mobile crane to lift the car off the battery!
But they did confirm the car has 2G.
Checking with mobile sevice providers the 2G cover where I live is bad. To quote one provider "Very poor signal outdoors, no signal indoors". This tallies with my experiences of 2G (and 3G) mobile phones when I first moved here. My understanding is that the TCU is (was) trying to connect to Carwings and with no response is staying on when I stop the car so running down the battery. I checked with Muxsan, (who had independently suggested disconnecting the TCU in a previous conversation), had one more re-read of threads on here and the OVMS pages (which is worth doing as it clarified and confirmed my thoughts) and decided to remove the TCU.
This morning (03/11/22) I removed the TCU. You guys with LHD cars have it a lot easier going in through the glove box space!
I have listed a few of pointers for those with RHD cars who may decide to follow me:-
1) The black panel over the pedals is retained by two plug clips (turn the cross head insert 90 degrees and pull out) and two steel spring clips. A stong downward tug releases the spring clips. It is not necessary to completely remove the panel as there is enough room to work with it dropped down.
If you look up and right under the dash you can see the black TCU box screwed to a steel bracket. It is almost impossible to release the plugs. Even if you were familier with the release catches it would be extremely difficult if not impossible. There are two socket headed srews but undoing them will not release the TCU as there are two more at the top! There are two black philips headed screws visible behind and above the release levers. These must be removed to release the TCU on its bracket.
2) The bonnet and charge port release is hooked into the dash. After removing the two bolts lift and pull back to disengage. (Mine was stuck (just been there a long time) and initially I had some difficulty getting it to move.)
3) Pull back the door seal just enough to uncover the end of the dash and prise out cover on end of dashboard. It is hinged at the front edge by lugs that engage in slots.
4) The corner of the lower part of the dashboard can be pulled away. I found it easiest to start nearest the steering column and lever with a wooden spatula. The release is horizontaly backward (Not down and back). You can then reach in over the top of the lower dash panel.
5) Two cables are clipped to the left side of the steel TCU mounting bracket. It is almost impossible to lever these clips out so I cut them. (Sawed through very carefully using a serated edge kitchen knife.)
6) Remove the two black philips screws holding the TCU bracket to the vertical support bar then the TCU on its bracket can be moved left and dropped down. It is then easy to unplug the three connectors.
The grey one is made of a brittle plastic and it is easy to break the release tool loop.
It is also deep and needs to be slid out parrallel to sides of its socket. There are no exposed pins so I just tucked the plugs up over the edge of the top half of the dashboard before reassembly.
I reconnected the battery and everything seems to work normaly with no alarm or warning light. I have not yet checked for error codes. I then reassembled the dashboard. The bottom right corner of the lower dash does not line up with anything and looks "wrong" to me but is covered by the end panel which fitted back perfectly.
When I disconnected the 12v battery a relay clicked so something was live, but I did have the drivers door open so it could have been something to do with the courtesy light. What sounded as though it was the same relay also clicked when I reconnected the battery.
I have fitted an Optima Yellow Top YTS 2.7 R Battery which is a straight replacement. Unfortunately it is a 38Ah but the next size, up 48Ah, is a bit too big to fit in a ZEO battery tray without modifications. This deep cycle + starting AGM battery is claimed to hold a steady voltage deeper into the discharge cycle, be much more tolerant of long periods of lower charge levels and sustain many more discharge/charge cycles than a "starting" wet battery. It is also claimed to be less affected by vibration which is probably a good thing where I live. We shall see.
Where I live the temperature rarely drops below freezing so hopefully with the vampire drain gone (fingers crossed) and regular maintenance charges, this battery will last a long time.
I am going to fit a (fused) pigtail through to the charging port but I have to work in the street and it has started raining (again
) so will leave this until the next dry day.
I hope this may be useful and will continue to update this thread with my experience of these mods, good or bad.