I can't remember whether I asked specifically about this subject, so I will risk asking again !!
I read, on the RR P38 facebook forum thingy, that both the diesel and petrol P38's use the brake light switch to help control the cruise control. I wonder if someone can confirm this ? My CC isn't working although the switch lights up. And I know that the diesel doesn't use the vacuum tubes, like the petrol vehicle. But, perhaps the brake light switch could be the problem ?