drayton digistat rf 601?

Hi, I bought one of these from ebay, supposidly brand new in box. I replaced a Drayton RTS1 with this unit. For 1 day it seemed to work then I got the same problem where the light goes green but the boiler does not fire up. I did a straight swap as the wiring block at the back looked the same, I have a N L and a wire to 3 for call heat. The documentation reads almost like you have to take the L out and put into the common block but I cannot understand why it worked then it didnt. Any tips? Is it going to be this resistor issue? As I said the termo puts the flame signal it talks to the SCR and this goes green but nothing happens. Any help appreciated,
Cheers