]I don't know this boiler but usually there should be a programmer on the wall somewhere. From the programmer you can usually select either hot water or heating or both. Often it is in the airing cupboard - but it could be anywhere. Also, if you have a room thermostat on a wall somewhere, such as the sitting room, or hall, turn that down.
If the boiler is old like my Glow-worm Sunrod then the only option to change from water heating to radiators is to switch off the heating at each radiator where it is not required. The general advice is that these Glow-worm boilers keep going for ever and you should think a lot before changing to a 'combi' and then say 'NO' since the latest boilers require lots of servicing and are often giving trouble it appears.