Neil,
When I had my '89 Plus 4 serviced/MOT'd by BHM earlier this year they reminded me (again) that although my tyres were fine, no cracking, etc, they were pretty old and should be changed for safety sake - 42k on the clock - just advice, not an MOT risk.
My front ones were 11 year old Yokos and the rear 13 year old Avons. The car was originally built with Avons.
I have since replaced all 5 with new original spec Avons and it feels much sharper, somehow, better in the corners, with all tyres the same make.
I may be old school but if the factory originally spec'd to a specific tyre then that is good enough for me.
By the way, my spare tyre was the original Avon - unused, 30 years old, no cracks....but it had to go, just in case.
Geoff