So I reluctantly noticed signs of damp in the ceiling. Turn out a mistake in fitting the roof vent 8 years ago means it (hopefully only recently) started to leak. Tempted to leave it, but the idea of having mold just above our heads for six weeks persuaded me to fix it properly. A few frustrating weeks later (and many discussions about sealing ceilings) and it is all sorted. Hopefully.
First signs of damp 
Finding the rotten, but dry, wood. 
The wood just cracked of easily. 
New piece of plywood cut and ready, with edges sanded down to blend in and sit on the original ceiling 
First fit to see if it will work 
Extra added wood to let the vent clamp better – this was the mistake previously 
Checking the additional wood is in place by peeking above the new plywood 
Ply all done 
Ready to spray glue and attach the carpet (or is it vanpet) 
Carpet on 
Mostly finished 
Final trim back on and lights fitted. 
And finally, new sealant on the top.
Yellow Van Mountaineering 