Help!! rear diff mount problem

[fivepot]

Help Have been renewing the gearbox and forward rear diff mounts on my Coupe Quattro this week and have hit a snag with the drivers side rear diff mount.
All the other centre bolts for the other mounts have come out easily after undoing the 17mm nut but this one is seized solid in the mount. (rusted?)
It really is stuck - have given it a good seeing to with a lump hammer and then tried jacking it against the weight of the car - but still no movement.Have a replacement bolt ready to go on with the new mount so not worried about damaging it
Any tips would be greatly appreciated.
Cheers,
Steve