Many thanks Privateer for your interest, comments and very useful photos. Unfortunately the suggestions you made did not work. The float moves smoothly with no evidence of any deposit build up. Everything was thoroughly cleaned and I even made a direct connection across the "switch" wires -...
Following on from the recent posts, I have a Cavendish CHS20 (grey sump and blue water container) which has been repaired 3 times by Dimplex (under warranty). Now it has broken down again and the warranty has run out.
The problem this time is that when the fire is switched on there is a "beep"...