Installing a Hive Thermostat
Your existing thermostat is just a switch - COM is the same a Permanent Live on the Hive and N/O (Normally Open) is the same as 4: Heating on (NO) on the Hive.

Everything else on the Hive you should be able to ignore - except the Permanent Neutral that the Hive needs and your old thermostat doesn't; this might be a problem.

There is a good chance the unconnected Grey cable is neutral, but I would investigate the other end of the cable, or put an electrical tester on it first.  If it isn't connected to Neutral at the other end you should be able to fix that in the boiler

The Yellow/Green is earth and should be at least connected to the backbox (although this might not be possible!)

CAREFUL THOUGH!!  This is all mains voltage stuff; make sure you understand what you're doing!