Glowworm Spacesaver KFB 50 fires up without water or heat called for?

Have had the programer replaced, motorised valves are working correctly (supposedly) Immersion tank thermostat also replaced. Bolier fires up when heat or water settings on programer are off. Also water very hot despite boiler thermostate being turned low (tried all positions, water just remains very hot and noisy, kettleing. Have now resorted to controling system by mains switch! Any advice please, apart from new boiler