Heating is controlled by
1) a clock and
2) a room thermostat.
Is the clock calling for heating?
Is the room thermostat calling for heating?
It is possible to override the thermostat with a link in the right terminals. See page 6, fig 10 in the manual. The thermostat should be connected as the switch marked "72" on the diagram, to terminals 5 and 6
Heating water is driven round the house by a pump. Can you hear if the pump is running whilst the clock and thermostat are calling for heat?
I rather think you will need a techie qualified to work on gas apopliances, I'm aftraid.