Hi again. Thanks for that information. I will have a word with my local Land Rover dealer about the cap. The new cap that I fitted should have been the correct one that they supplied, but will check with another cap?
Some other bit of information about the vehicle is that the owner had not long done a trip from Bristol to Plymouth, towing a large boat and trailer. He said he had no fault with it on that trip but did say that he had not checked the water level before he did the next one.