With the recent talk regarding tyres in the forum, I had the slightly curveball thought of Blockley tyres.
I am aware of Blockleys because of my interest in Vintage (20s) cars. Proper performance tyres for old cars (Bugatti, Bentley, etc) have always been a problem, and from all accounts Blockleys have taken that market by storm - it is worth a visit to the website to read the interesting story.

Anyway, they are moving into performance tyres for newer cars - see the sizes

Blockley Tyre - Vintage Racing Tyres (scroll to bottom of page for post-war sizes..)

it occurred to me they may suit 15" XJS wheels , and the price didn't seem too bad to me, for a very high quality tyre.

Check it out!

 

Thanks for the link, those Blockley's are good looking tires. But a bit narrow for the XJS, no?