my tenants potterton prima needs rebooting every morning?

Please please can anyone help. We've got a tenanted property (only 13 years old) with a potterton prima system boiler. For the past year every morning the tenants have to get up and manually turn the boiler on again to get the hot water to start. I have now spent thousands with various plumbers trying to get someone to find out what the problem is but no joy. The have fixed the pressure reducing valve which was leaking and the diverter valve, drained the system down, a new pump, power flush, new valves everywhere, alternator replaced and still no luck!