Hello Ken,
Greeners is right. The only BMW power plant fitted as standard in the factory was the M47R 2.0 diesel engine to the Rover 75. The 2.0 petrol engine was the Rover originated KV6 series which is belt not chain driven.
I have tried to look up your VIN number in the build records to see what your car is listed as but it's one of the later Cowley built cars which is missing from the database, so I can't.
However, looking at your quoted engine number and comparing it to the engine number codes for Rover 75 which you can find by opening the pdf on this page
Here you'll see that your 3rd prefix code (M50) does not match the Rover 75 codes listed which is odd?
Can you post a picture of your engine, we can then identify it as the 2.0 belt driven petrol like this one
or the 2.0 diesel chain driven one like this
