Thanks to those who posed here (DandM Dame and mas.sheraton) I have managed to get my Nest 3rd Gen working.
My problem:- failure to establish connection between heatlink and thermostat (H72 error). This persisted despite many reboots, units being next to each other etc.
Solution : Use the wired setup, connecting T1 and T2 of heatlink and thermostat) and completed the setup steps without a wireless connection. This allowed access to main menu and the factory reset option. Started working immediately for me after this, would recommend you also do a heatlink reset (hold button for 15 sec) just before factory reset on the thermostat.