3 wires into a 4 wire digital thermostat

[RossyBeel]

I want to upgrade to a digital thermostat but the manual says I need 4 wires when I only have 3- brown blue and grey. The thermostat says L1 N1 N and L. Please tell me where I put the wires!

 

[JohnD]

start by telling us which make and model it is

a photo of the connections will help

have you got the instructions?

 

[MeldrewsMate]

In the absence of any other information, and the fact that you're willing to modify mains wiring, I assume you have access to a multimeter. If so then you will probably find that your N and N1 terminals are electrically connected - your multimeter reading will confirm this.
You most probably (not certainly, not definitely) have brown = line (230vac), blue = neutral, and grey = switched live.
After these are proven to be correct then try brown to L, blue to N. Leave the grey unconnected. Stand back and switch on. Does the thermostat light up, switch on, or in all ways show signs of life? Does L1 switch between 0v and 230v with thermostat setting? If so connect grey to L1 and retry.

*NOTE: all the above is only valid if you are competant to work with mains. If not, don't touch.