I would say your asking price is quite fair.
Mileage and owners isnt the biggest factor these days, mainly condition etc.
As said cat c could be something really simple, history of it would defo help.
Mine is K plate, Nouveux Red, leather, air con, 18s etc, I would value that around £8k.
Same car with cat c I would say around £6k, so as said before fair asking price
Good luck with the sale