OK - update: taken the BeCM cover off, and the top PCB is badly fried adjacent to the long grey connector, the power tracks are burnt off of the board and the glass fibre is burnt so badly it's delaminating + the long grey socket above is heat welded to the plug so I can't unplug it :-(
I can't see the circuits fed from the grey plug (C325 / C1283) being the cause as this feeds the rear lights / Indicators / door locks / aerial amp & RF receiver (according to an the 'Demystifying the P38 BeCM' article), although it would explain why the rear doors no longer unlock! The ribbon cable closest to the grey connector is also burnt, will try unsoldering one from the spare BECM logic board.
Looking at the damage to the PCB, a massive amount of current must have been pulled from the board, but I can't see anything obvious that would cause this - will have a close look at the good power board this weekend and try and work out which socket those traces went to.
I've cut out the grey socket from the board so I can remove the BeCM from the car - eBay to the rescue with a seller have a set of plugs for sale (rather than hack the loom from the 4.0 HSE apart, as I may need to perform a major swap if using the 4.0 BeCM is my only way of sorting this out without going broke).
I read somewhere that the 'power' board's are all the same - this looks to be correct comparing it to another I already had (bought originally to look at how the current sensing worked re bulb fail as I wanted to fit LED bulbs - still on the ever increasing 'to do' pile), so will try to swap it over once I've worked out where the current drain came from.
With all the rain we've had, the drivers side front carpet is soaked again - I still think it's the water shedder on the drivers side door (or should I say the lack of one) allowing water to run over the door seal onto the carpet. I'm reluctant to fit the new shedder I have until I'm sure I don't need to take it apart again.... I'm >90% sure it's not coming from the heater vent since I sorted that out.
Oh, on a better note: I've picked-up two second hand (but good) Optima 5.5 Yellow Top Starter / Deep Discharge batteries (eBay - £45 each, were used for backup generator starting) - one now fitted ready for the next attempt (seller still has some if anyone is interested - collection only in the Southampton area)