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Just a guess based on light switches but common usually refers to a switch line which in theory carries voltage between 2 way and intermediate switches. So common in this case could be used to wire up more than one nest or thermostatic device, allowing any one of the devices to call for heat.
Which could be why Tango130 only has the call for heat signal wire.
Airhead
Interesting, I have a Worcester Bosch combi, am I right in thinking that I need to locate the boilers "call for heat" and "common", then wire them (along with power) into the heat link?
There's no connection to my thermostat other than the 12v power.