I am trying to build 196m north of Norilsk Base and 152m south of the mission crate that is also in the forest. It still throws up the 140m message.

Has the whole of the forest terrain north of Norilsk been tagged as 'city' terrain ? Or does it count rocks as 'city' terrain ? Or my own vehicle ? Or disused fireplaces / flares ?

I am at a loss as to why it won't let me build here ?

Sometimes objects like wrecks or small sheds count as a town. Look around and see if there are any objects that may be messing you up.

Yes, any small outbuildings, wrecks on the coast (as paint suggested) and also the power stations count as part of a town. The castles also count as towns now so you can build there anymore.

I died and logged back in and it worked, same spot as before. Sometimes Arma is annoying.

If it does it again if you keep building there try to relog and it may fix it.