OK you have an S plan one valve for CH and one for HW unless you have another room thermostat that you havent told us about, you will feel a small lever on the end of each valve, when the HW and CH calling for heat this lever should be able to slide across with no resistance , with them satisfied or off the lever will still move across but you will feel resistance, next time the CH doesnt come back on , dont touch anything and go to the CH valve and slide the lever across and see if it has resistance, it might suddenly move across as you slide the lever