New Danfoss RX1 won't work?

My old Danfoss RX1 stopped working - no green prog light when Prog and CH1 pressed together for 3 seconds, no green light if Prog pressed on its own, but a red light and boiler fires up if press CH1. I have replaced the unit with a new Danfoss RX1 (not replaced the back plate) and its doing exactley the same. Any ideas please?