Front cross member bolts,

[Simeon]

Just replace the two front crossmembers, they're very cheap secondhand, I've got a pair for sale for a tenner.

[rob.043]

Could the excess movement be worn CV joints? (Im wondering about this for my own car too, what play/wheel rotation is acceptable accross the axle?)

Also consider the other two engine mounts (rear mount and gear box mount), if they are old and fooked, they could probably do with replacement too to reduce engine movement.

[beechymonza]

No i dont think its worn C.V joints,
This like a movement as if other engine mount or gearbox mount has probably gone,

[rob.043]

Similarly, could nackered/loose diff rivets be partly to blame?

Probably is the mounts.

I believe it is recommended to get the G60 spec rear engine mount, and the 1.3 model gear box mount. As these are the solid rubber ones of the best design. All will fit.