changing central heating thermostat?

I currently have a Honeywell stat for the central heating and have been given a digital one to replace. can anyone tell me where the wires go.
The existing Honeywell has 2x wires blue to No. 1 and brown to No.3.
New stat is a Drayton Digistat+ with 4x terminals.
OFF, ON, COM, N. Not sure where the wires go...