Insurance companies are a fcuk ing nightmare in general, I have max NCB on three vehicles. had a no fault claim in 2023 (parked vehicle hit by a truck) whole thing covered by a security camera. My insurer tried to say it was a 50/50 claim.
That they said removed my NCB discount on all 3 policies.
They did actually put it all right, paid my claim in full and didn't effect my NCB on any policy, but only after a letter threatening from my solicitor.
If you had 1 vehicle and then added another then in most cases the existing NCB will NOT apply to the second vehicle. But as Misterriceman said each insurer seems to make up their own rules as they go along.