I have solved the problem myself and if this info is of any use I'll pass it on. Being a two year old caravan I was wary of doing anything to invalidate the 3yr warranty. After a long wait and unhelpful emails from Thetford they sent me a download showing how to get the pump out. I turned the impeller with a small screwdriver....still no action. I gave the pump a gentle tap.......still no action. I gave the pump a less gentle tap and hey presto the pump burst into life. Total time to do about 15mins!! If you have this problem I suggest this order.
1. Check fuse in cassette toilet area. 3 amp
2. Check fuse in main switch panel
3. Check switch either with a test lamp or meter gauge, or watch if bathroom lights dim when switch is pressed as that is normally a sign that the switch is working and drawing power.
4. Remove the pump (a very simple job once you have the info which i can direct you to) turn the impeller with a small screwdriver.
5. If still no action tap the pump until it spins.
Ivan Cox
April 2008