intermittent hot water - Valliant Combi?

Hi, I have a Valliant Combi (about 10yrs old, no model number that I can find) that sometimes just doesn't give hot water and sometimes does. It is like the boiler doesn't always recognise that hot water is being demanded - no lights go on, nothing fires. Occasionally when I turn the whole boiler on and off it comes back on and sometimes it doesn't. Sometimes I don't do anything and it comes back at some point.

Any ideas? When I called the plumber out it was working again so he couldn't trouble shoot.....

thanks