central heating not turning off

I have a Gloworm Flexicom 18hx boiler, Drayton T721R Programmer, a Drayton MAI/679-3 3 PORT VALVE and a Drayton RTS2 thermostat and a Drayton cylinder stat. The programmer, 3port valve and both thermostats are all new. The hot water system is behaving correctly, switching off with the...