During a visit to storage over the weekend I thought I would make sure the 13-pin plug was OK and all the van lights were working after our problems previously.Took the plug apart and made sure all the connections were tight.
Everything was OK but as I put the green cover cap back on the plug I noticed it was pushing the O-ring seal out of its groove and had presumably stretched it.
After a post on CT about sizes ( trying to measure an O-ring with a Vernier gauge is like trying to strike a match on jelly ) some kind soul informed that they are 40mm internal diameter with a cross section of 1.5mm so it may be worth making a note if ever you need one,saves buying the whole plug if it comes adrift.
Ordered a packet from here :-
When I replace it will put a drop or two of super glue on it to keep it in place.