We got this excellent spot from Park4Night. It was already night time when we got there, and it was pitch black, apart from a lit cow shelter nearby. We could see a few other vans and motorhomes, so felt pretty confident we were safe and in the right place.
As usual, by the time we got up at 10ish everyone else was gone! And cars of dog walkers had started turning up, presumably to visit the castle, and I think there is a popular walk nearby. No facilities, but can’t complain for a safe and quiet spot close to the motorway.