Thank you all for returning my sanity.
For 3 long years, I argued with my supplier and main dealer that the cruise control did not work. They always said they could find nothing wrong with it.
Usually you get about 50 miles and then it decides it has helped you enough and packs up.
As with the water in the fuel filter light, I now consider them a quirk of my disco's character. (along with the sunroof that opens when it feels like it).