So close, yet so far... Just went and had a look behind the thermostat and indeed, there is no blue cable there... The whole bundle is also in a grey outer sleeve, whereas at the boiler end it's in a white sleeve, so guessing there's a joint somewhere else in the house, perhaps where the upstairs & downstairs thermostats split off?
Anyways, given that at the thermostat end, I have just grey, black and brown and earth, could I, in theory:
Brown joining Hive L & Hive 1 together, ultimately to Boiler L
Grey connecting Hive 3 to B1 at the boiler.
Black connecting Hive N to Boiler N.
This would mean Bk2 (which I think is a live of sorts) isn't connected.
My only last concern is that where is the blue cable in the boiler, currently N, connected to, as at the thermostat ends, nothing matches that colour... Might be on for a long run of wire and a multimeter...