Update - 13/11/06
Service Engineer called today from Glen Dimplex Home appliances (Belling's approved service Engineer). He was here for 30 minutes and knew exactly what it was. The problem connected with my 'E5' Error was a 'faulty pressure switch', the device that controls water flow. It was accessed from behind the front panel floor plate and was an orange/red piece of plastic about 2" in diameter with a number of electrical contacts. He pulled that out, put the replacement in, switched on again and hey presto I'm repaired.
Not cheap £79.90, but he didn't charge for the pressure switch and now I'm working again.
In the end, the waste pipe, cyphon and filters were not blocked which was suggested by the manual.
If anyone's at their wits end, you can contact Glen Dimplex on 0870 444 9919.