Any as far as I'm aware, they don't ask when you get a quote. In the event of a claim they will see it was a cat C and any pay out will be lowered accordingly.
I also ride a Cat D bike , wasn't asked the question, the Insurance broker or company could have performed a check but didn't so it's only at payout stage I'd get a reduced amount. I guess the challenge would be how much of % reduction in payment would you accept, For Cat C you may be offered 50% less than book , be interesting if anyone has any real example rather than guess work.