Disco 2 TD5 random flat battery

[miniman]

My Disco has a random habit of having a flat battery. It seems to drain overnight, recharges fine once started from a boost pack, and is then fine for weeks, months at a time. Then it will go flat again. Have been consciously checking all doors are shut, interior lights off etc.

Any ideas? Wondering if there's a similar issue to the BMW final stage resistor (hedgehog) problem which keeps the a/c fan running.