I've just had the same problem on this model. It is due to a bad connetion to the printed circuit board (pcb). The pcb turns on the red light when it is activated by the floating magnet in the loo cassette as it fills up.
If you take out the cassette and look into the space, you will see a green pcb at the back with 2 wires going to it. The wires are connescted to the pcb by a small plastic connector. Gently pull the connector from the pcb and clean the contacts on the connector and the pcb contacts with some fine sand paper to get a good contact again.
I spoke to Thetford UK and they advised me to do this. If this fails, give them a call and they will send you a new pcb free if still in warrantee.
Neil Plested, June 2011