Sorry can't help on that one but go back in to thetford site follow instructions below to access the download list of their help repair pdf files.
After clicking customer service on the bottom of the list is 'Ask Thetford' you can send them your problem if you haven't used that access yet then continue to the download page.
I've included Australian site details as requests have come to me for C2 repairs from as far as that I've done up a file now on the C2. but now forum prevents adding my email address for contact but its on page 17 of the forum in the reply to THETFORD TOILET BENCH TYPE NOT FLUSHING question.
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)
NEXT PAGE SELECT REPAIR INSTRUCTIONS (left of pic)
NEXT PAGE SELECT BUILT IN TOILETS [top right of pic]
NEXT PAGE SELECT C2, C3., C4 or select your model range [in your case obviously c400] a flashy one too. Mine's the oldie like me.
NEXT PAGE PDF Files YOU ARE NOW IN PDF FILE LIST
download any or all file up keep in c/van
Bob, May 2009