There is nothing like answering one's own question. This may be of use to others one day. The original problem was that part of the plastic framework that holds the base of the bracket onto the copper pipe had cracked away - plastic fatigue. This resulted in a poor contact with the valve underneath. So the whole unit was removed and checked over in the following manner. 1 - checked that there was an electric supply to the unit - ok. 2 - opened up the micro switch cover carefully to expose and remove a simple switch plus, and be careful here, as you pull it out, because two brass slides and a small spring will fall out of a tube inside. They fit together I believe with the larger slide at the base, the spring in the middle, and the smaller slide at the top. The switch was easy to check with a buzz / multimeter and was ok. 3 - So looking online I find that if one undid the top unit ring and lifted the head off, inside was revealed an inset screw that could be adjusted anti clockwise but not more than half a turn, until on assembly of the whole unit the switch would be heard to click on and off. Be advised that before assembly the settings on the top unit have to be rotated to the 5 marker. Amazing having repaired the base as best I could, and having put it all back it worked!
Tony Hodge
November 2009
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix home heating systems.