We have taken the plunger out and discovered that a plastic bracket at the bottom of the plunger has broken but there seems to be water under the cassette when you take it out which is running down the sides and getting into the wall beside the toilet. We did overflow the toilet at the inlet a few weeks ago but looks like this may have been something that was happening before we bought it.

Julie, November 2016

I had this problem for two reasons - firstly make sure that the cassette is fully home and that the blade is open when doing the
business so to speak, as the round seal around the opening can sometimes not seat/seal completely ( it is fairly easy to change these however and not expensive) also try using some of thetfords seal lubricant on this seal to keep it flexible.
Bigger problem that I have found with the thetford is the flush pump. When you lay the van up for winter please make sure that you fully drain the flush tank - there is a drain tube for this and what I do is use a CPR mask on the flush inlet and blow to clear everything from the tubes and pump - one year I didn't do this and there was a sharp frost and it cracked the pump and allowed flush to dribble out into the cassette compartment. It's about a £35-£50 part and a bit of a fiddle to fitnut if you're a bit of a diyer not too hard.

Glen, May 2017

Go to www.thetford.europe.com.
Model, if known, would have helped.

Either Select your model by viewing pictures of type it is
Contact Thetford using online form ask what could be cause.
Meantime try this on the cassette is a rubber seal around the blade opening.
This may be letting flush water to seap past when blade is opened and bowl contebt released and flushed.
To test: talc or chalk powder. Spread around the edge of the rubber seal about 1/4 from it. Replace the cassette 1/4 fill the bowl with water, leave four 15mins open the blade if any water remains, CLOSE the blade then remove cassette wet powder means its likely either wrong seal(they are year dated according to caravan age and toilet model)or or the sea is old and not doing its job. Always, always check cassette blade is CLOSED before any attempt to remove cassette from housing or you'll snap the blade operating arm.

Bob Uk, December 2016