Hi
Just thought I would update my experience, which might help someone:
1) accessing the fuse box on the passenger side is a nightmare for an amateur like me. However, with a pair of log-nosed pliers, I was able to check the relevant fuse. I am not sure if the difficulties of access were compounded by the additional electrics for the camper conversion. The manual I have says contact your Toyota dealer, so Toyota clearly know there is an issue.
2) I got no help at all from my local Toyota dealer - refused to engage. I suspect it is a bit hit and miss as to whether they can be bothered
3) I found an autoelectrician in Shipley, Luke at Drive in Service Centre on Otley Road. He diagonosed what had gone on in 10 minutes:
a) nothing to do with key or immobiliser
b) when the AA had recharged my flat battery there had been a usual surge as all of the electrics power up at the same time
c) somebody had previously done a repair on the cable from the battery to the small fuse box under the bonnet
d) the battery was fine but the surge had shorted the cable and blown one of the fuses - this accounted for why the dashboard lights were lighting up, but otherwise everything electrical was dead.
e) I was told that UK Toyota diagnostics equipment can read all Toyota vehicles. Toyota could have made this assessment for me.
So, back on the road, no issues, but now very careful to turn off lights (and on auto). There is no warning signal if I leave my lights on.