Hhmm.. New thermostat doesn't use batteries and no display.

[wimmer]

I decided to change my digital thermostat for one with a backlight, only problem is there are only 2 wires (plus a blue and green/yellow taped up). When I connect the brown to the L and the black to L1 on the other side of the switch on the diagram, (as removed from the old thermostat), there is no display. As the new one obviously needs constant power for the display, I obviously need to connect something else. There are 4 terminals on the new one N1 N (AC IN) L L1 - thanks anyone.

 

[bigads28]

You need a neutral. If the old stat didn't use it, it may not be connected. It will need to be traced out and check by a spark or heating eng, if you aren't sure of what you are doing. You can do real damage to the components in your system and boiler by getting the wiring screwed up.