Thanks for all your help guys.
Finally tackled it today (wanted to wait until the other half was away for a few days in case it didn't go to plan).
All done and dusted in less than an hour. Everything connected and working like a dream.
Just wish I could've run a couple of wires from the Heat link to the thermostat, but got 2 door frames and walls in the way. Or replaced the existing thermostat and changed the wires, but wasn't possible. So at the moment the thermostat in on a table in the lounge. (I don't fancy having conduit on the wall).