Well, that was a bit expensive (£75 + P&P) and rather disappointing.
The replacement fusebox arrived this morning, I have changed it over and while it is all "char-free" with a full complement of un-burnt relays and shiny fuses it has made absolutely no difference to the situation.
As soon as the engine is running the cyclic clicking of relays begins and the low-fuel and overtemp alarms light.
As I have disconnected the battery all of the windows want re-setting and I notice that as we go through the relay-clicking sequence the power to the windows is turned on & off too (but I would expect that if there was a "reset" signal being sent or generated from somewhere).
I have no idea what is going on.
Maybe there's a clue in 3rd line above ?? What does the alternator output look like ?