I can try to address some of these questions separately, the thread is long but I think all the info is here...
What do I need to check for that my seats have heater mats?
There will be an additional large plug coming from the seat harness (if you compare it to the harness of your non-heated seats). Otherwise you could start pulling off the upper seat cover. The bottom seat cover is too much work but getting enough of the upper seat cover off to see the mats isn't that bad.
Aside from ringing dealer but how do I know whether the car originally has heated seats option?
This should be no problem if you have the VIN of the original car, easier than checking plugs or pulling seat covers. Just call and say you're looking to buy that car and want to verify.
Which module in VCDS do I interrogate to find which BCM type I have?
Module 09 (J519) Central Electronics is the BCM... post it here, someone can probably tell you if it's low/mid/high
How do I check the heated seats option is there or how do I activate if it was not factory optioned?
In VCDS select Modules > 09-Cent Electronics > Coding > Long Coding Helper... scroll through for the heated seats option
You can also physically check for required pins on the BCM as described in post 20 in this thread... this will tell you if your BCM supports heated seats
Should the wiring be present running from Dash console to the underneath of the seats irrespective of whether heated seats were optioned or not?
No, unfortunately.
If not where would you recommend sourcing the appropriate loom?
Bought mine from Kufatec, but you can also find them on AliExpress... search something like "Golf 7 Heated Seat."
How do you connect it to the existing loom that plugs into back of Climatronic unit that house heated seats switches?
Harness connects to the BCM and fusebox, left footwell area. The Climatronic just has the buttons, but those buttons tell the BCM to power the seat heaters.