Hi, I had exactly the same problem & I hope I can help. What you need to do is ...
On the bottom of your wireless room thermostat is a small screw, undo the screw then pull out the bottom of the thermostat and lift it off so that it is now portable.
Take it to where the reciever unit is in situ, usually next to your boiler then on the back of the portable thermostat are two buttons, one says Learn the other says Test.
Firstly push the Set button on the reciever unit and immediately use a screwdriver to push the Learn button on the back of the thermostat unit.
The orange light on the reciever should start to flash, then use the screwdriver and push the Test Button on the back of the thermostat unit and the orange light should turn green.
Thats it now the two units should be married up to work wirelessly. Then just remount the thermostat back on the wall and secure with the small screw.
Now on the boiler itself switch it on with the button top left then press the Restart button & your boiler should fire up. Goodluck.