Rewiring a Nest & Heatlink

Xeorix

I've recently had a Nest installed in my home, however unfortunately the engineers didn't listen to what I asked and have put the Heatlink in the wrong place. Basically, they couldn't be arsed to extend the wiring, so now my Heatlink is in the place of my old thermostat and my Nest on USB..
I plan to put the Heatlink by the boiler, and run the 12V cables from the Heatlink back to the Nest where it should be (where the old thermostat was, and where the Heatlink currently is)

From what I can gather, there were 5 cables going to the previous thermostat, as the existing wiring has just been wired into the Heatlink.

What I need to establish is:
- What type of cable would have been used to wire in the thermostat to the junction box? Looking at this guide, it appears to be a 5 core, or is the 230V power separate?
- What should the spec of the above cable be?
- What is the best way to extend this wiring? Junction box for 230V and connection blocks for the rest?