Hi Chris, Bilt Hamber despite strange name are one of the bigger car detailing product specialists. Good products but not always cheapest – just seek out a keen price.
If you want to check out some alternatives though – google.. ‘Auto Express Review Clay Bars’ – should bring up 2017 reviews..

Regards, Robin
Ps I do have a 1987 3.6M Cab aswell as the XKR..