problem with Holsted Finest central heating boiler

[BARNFRANK]

The safety cut off switch keeps coming on, this cuts off the hot water.
The fan contiues until we turn off the power.
The only way to get the hot water back on is to turn tha power off and on again.
Biggest problem is when the gas engineer comes around it never happens.
Anyone got any ideas what could be wrong.

 

[bengasman]

Does it NEVER happen during heating?

 

[BARNFRANK]

No not really, only happens when we try to use the hot water.

The service engineer does not know what to do, because it never happens when he comes around.
We always reset the hot water,so that we can get some hot water, by turning the power off and on, this is mainly to stop the fan, which does not stop.