One of the Taxi guys had exactly the same weird fault on his Omega, started fine, and went flat over a day's work, tried and tested everything, finally left it at main dealer running, and they discovered one of the battery cables had corroded internally, and was heating up when in use, that flattened the battery in a days running no problem, they had done batteries and alternators beforehand to no avail. easy check, just attach jump leads direct fromthe battery to car and bypass the cables, to see if you get better volts than without, at the ends, ie back of starter and engine block.Or just leave it running and see if they heat up.
