Fault with Ravenheat Combi boiler (heating goes off)?

I have a Ravenheat Combi boiler which provides hot water ok. However, when the central heating is switched on, all comes on ok until about 1/2 hour to 40 minutes later, when radiators go cold, and hot water not available. The pump sounds like it is still operating, and the burner appears to be on (appropriate light is on on the control panel). If the boiler is switched off manually, then back on again, then heating comes back on and hot water is available again - Only for the process to repeat itself in 1/2 hour to 40 minutes time.

Any ideas what may be causing this, my gas engineer seems to be unable to diagnose.....