Vokera 24/96 RSE Flowmatic Burner stays on?

Hi, I have the above model Vokera boiler, however the main burner will not shut off, so it is heating the house constantly, even when set to timed with the clock set so that the heating should never come on. The only way to stop this is to turn off the main power to the boiler, however if this is left and then switched back on the main burner again ignites and will not turn off.