Check the Hot Water Diaphragm, Vokeras are renowned for worn diaphragms
www.elanar.co.uk, May 2009
You need to say what model it is. Different boilers have different ways of achieving the same thing. But it sounds as if a microswitch that ought to be activated to tell the boiler thare is hot water demand is not doing its job, either because it is broken or because it is not being operated due to a faulty mechanism.
