Hi, oops! major overflow of (fortunately) clean water. The flush seeps slowly all the time, it is the variety which takes its' flush wter from the main system. The flush button operates a valve (? which then flushes the bowl and stops. Unfortunately it doesn't quite stop and over the day/night when not in ose, overflows. HELP! Thank you.
Thetford part number 23709 looks very like your likely culprit from the diagram I have pulled up it may be that you could open it up and get a seal at a plumbers merchant but there are no seals listed from thetford.
However the whole valve is under £40 so not too bad.
Obviously confirm this is the fault before ordering but from what the diagram shows there's not much else it can be