Try their website.
I am assuming yours is the two piece portable. ie: flush tank on top cassette below seperated by catch at back. I would say it's the valve seal causing it but if in doubt send your question to thetford on 'ask thetford' and enter portable two piece for type if that's what it is.
UK www.thetford-europe.com
Australia www.thetford.com.au
Australia site has stockist list.
HOME PAGE SELECT ENGLISH {if in Australia forget this selection
NEXT PAGE SELECT CUSTOMER SERVICE (tope line)
NEXT PAGE SELECT USER INFORMATION (left of pic) also 'Ask thetford' is on this page.
NEXT PAGE SELECT REPAIR INSTRUCTIONS (left of pic)
NEXT PAGE SELECT BUILT IN TOILETS [top right of pic] OR PORTABLE FOR TWO PIECE TOILETS
NEXT PAGE SELECT C2, C3., C4 or select your model range
NEXT PAGE PDF Files YOU ARE NOW IN PDF FILE LIST
To direct link to the Thetford download site type in the following
www.thetford.eu/web/show/id=51498/langid=42
Don't bother for Thetford fridges they tell you for safety reason to take it to a service centre
Bob, June 2009