Peter :
That is the correct lead - the problem is that your amp has no earth connection - I would suggest connecting the earth to a convenient earth point - a radiator pipe or similar. You cannot access the rear channels as you would need a four channel amplifier - I wouldn't bother!
I don't think the ground lead will be a problem. Since the BG6000 sends the audio out at line level, the unamplified signals prone to hum stay all inside the Beogram, grounded or not. Try just leaving it unconnected.
Please leave the radiator pipes out of the equation. Using them for grounding can be an electric shock danger if anything goes wrong in the equipment.