boiler problem - hot water works then cuts out after a few seconds
 

hi all
we have a vokera compact combi boiler and the hot water isnt working
properly....
basically, if you run it really slowly, then the boiler fires up, and then
you can get hot water and turn up the flow
if you just turn the tap on as normal, the light goes red on the boiler and
it has to be switched off/on again
any ideas anybody?
thanks

boiler problem - hot water works then cuts out after a few seconds