right, 1st things 1st, make sure the battery is good and that its being nicely charged when engine running, at least 14.2 volt (get ya self a digital multi meter for a tenner) and have a look at the earth cables, batt to chassis and chassis to engine!
the warning systems on a P38 are voltage specific so a dodgy battery or undervolt will throw up all manner of fault warnings, i recently had an issue with a 5 month old battery with a dead cell, i had g/box fault, abs fault, srs fault, flashng lights and gauges up/down on there own! just because of 1 bad cell in battery.
i dont think the trouble is your BeCM waking up over night as you havent mentioned any problems of a flat battery nature! next get it to suitable garage with the relavent tools to reset the fault codes and see if it comes back, (most indies will charge £40 to plug it in) any fault code loged in the memory can be cleared and as you dont seem to have any issues with the drive train i wouldt listen to the people who said its not worth it!
get it cleared and if it comes back then you will know to look further!
as far as there concerned it requires a new transfer box and there waiting spanner in hand and till open to do it, if after the fault code is still there you can bet your life you wont get ya money back...........