Hello Sime 80E owners I have had the same problem with my boiler not delivering hot water on demand and also no central heating ! both of which were fixed in the short term by switching on and off I have just replaced the divertor valve supplied by ezypart.co.uk cost £48.00 all in delivered to the door next day excellent service and very helpfull with regard to the remedy to the problem.
However the boiler was still not working correctly so have also replaced the microswitch which is activated by the divertor valve on demand for hot water,job done boiler now ok.
another check to do is the ignition electrode gap which should be 4.0 mm with a tollerance of 0.5 mm either way if gap is excessive boiler will not light
Neil Johnson
December 2004