What is wrong with my central heating?

Hi
My central heating and hot water will not turn on when requested. The electronic time is set to 24 hrs and the thermostat is turned up to 35 degrees plus but nothing. The boiler on light is lit but nothing. All other lights stay off and I do not get any hot water and central heating. The engineer has visited and changed the electronic board and the solanoid but its still not working.
The boiler does eventually light after maybe 1 to 1 and a half hours but he central heating will only work if the hot water is switched on.
Can anyone help? Engineer due again tomorrow!!

Cheryl Schofield
May 2012