An update:
Olga had some wear damage on the driver's seat base
I swapped the left and right cushions, to even out the wear, and repaired the damaged one. It's not perfect, but looks fine in the car
(Rather than go through all the process here, I have done it as an article and asked the Forum Admin to post it)
I used Furniture Clinic products, including glue, filler, surface prep and colourant matched to a sample I posted
https://www.furnitureclinic.co.uk/
The colour match can't be perfect on this type of vinyl, because the original has a base colour with a slight dark 'dappling' printed over the top. A repair product can't replicate this exactly. My repairs aren't invisible, but all in all it's good enough for a satisfactory repair
All for now, more soon
Cheers
John