The lower rubber that sits between the washer on the shock rod and the body contains a plastic collar (integral to the moulding) which wears, causing clonks and knocks. Also it is not uncommon for the top mount lower washer and metal bush to corrode onto the shock rod, making them unusable. I have also had a couple of top nuts shear off rather than undo.
The kit contains:
1 x lower bolt
1 x lower bolt washer/cup
2 x upper washers
1 x top mount lower rubber
1 x top mount upper rubber
1 x metal bush insert
1 x lock nut
The kits are essentially the same saloon/estate but the metal insert bush is a different lenght iirc. Certainly the estate kit is a different part number. Estate kit is apparently no longer available but the components are all available from GM.
Aftermarket options are limited to the upper rubber bush.
Hth