Update on my non-starting Vel Satis:
Auto electrician examined the car this afternoon.
First time the key was inserted, the whole dashboard and all lights etc flashed on and off repeatedly – pretty scary!
Tried connecting a portable jump starter pack. The fully-charged pack went from green (full) to red (empty) in seconds! Car wouldn't start.
Tried jump starting the Vel off another Vel and the voltage (according to the meter) of my Vel dropped to zero!
Then tried jump starting the Vel off a fully-charged battery. Again the volt meter said my battery was zero!
Various other tests were run which seemed to hint at the electrics being dead but not due to a dead battery i.e. worry set in.
Then the negative terminal was examined more closely and it seemed a bit loose. The two connections were disconnected from the negative terminal and then reconnected, ensuring they were tight. All battery tests registered 12.50 volts or more.
Tried starting the Vel. It spluttered on the first couple of attempts, then burst into life. Only one headlight worked for a few minutes, but then the other came online again. The TPMS said all was ok but didn't display any pressure figures. Eventually that righted itself too. So entered the radio code, went on a 20 minutes drive and all seems fine!
After the 20 mins drive, the battery measured 14+ volts with the engine (and alternator) still running. Engine switched off, left for a while – measures 12.39 now. What on earth?!
Fingers crossed he starts the next time I try!