Could this be caused by an internal leak in the oil cooler?? Could it be worth a try?? This one or similar:
http://www.ebay.co.uk/itm/Rover-75-M...item3a7f018175
If the oil hoses connected to the cooler have the same connector in both ends, you can try to bypass the cooler by connecting the oil outlet directly to the oil inlet on the engine side with one of the hoses to see if the problem disappears.