All sorted and thanks again for the previous tips, I tried as you mentioned even loosening the beam mounts which helped to get the diff mount to sit flush with the floor cross-member, I then put the front fixings in but left the rear fixings out to see what happened when I tightened the beam back up, and it did start to pull the diff mount down at the back again.

I was not happy with putting this much strain on the diff mount, so my options were ether shim the beam mount or shim the mount to floor cross-member but I opted for shiming the mount to diff section, I made a shim out of a piece of aluminium plate the same shape as the two oval plates and machined it from 4mm to 1.5mm and placed this between the diff and mount mating suffices, which made it meet the cross-member squarely.

I probably read to much into it and I know most people would of just tightened it up as it was, but thing like this play on my mind.