I have a worcester bosch 30cdi boiler fitted with a dt20rf programmer & digistat. it is 18 months old and recently has developed a fault where the receiver & rf stat seem to lose communication and the heating shuts down. i can reset it by removing the batteries from the stat, reinserting them and then resetting the receiver. is the fault in the stat or the receiver? do they have to be replaced as a pair? thanks for your help.