how can I repair/service Landis-Gyr thermostat?

Landis-Gyr RAZ13/2716A Thermostat is fitted to IDEAL Concord 'C' Series 3 Boiler, requires repair or replacement. It 's working but cuts out to soon so hot water is not reaching the upper floors radiators. Plumber said flushing system will not solve problem as boiler doesnt run for long enough. Help please.