Central heating won't start up on Sime Formula 80?

From a cold start my Sime Formula 80 will not start the central heating. The boiler will fire up for a few seconds, then stop. The only way I've got round this is to manually start the boiler, turn on the hot tap for at least 10 seconds, turn off the tap, and the central heating usually keeps going. Can anyone suggest a remedy for this problem.