And the update is...
I am happy to report not currently trapped between worlds at Gardermoen.

Actually arrived in to bergen, where it is currently raining (surprise surprise!)

Due to the inability of britain to run public transport I arrived relatively late for check in at LHR and there was a queue in front and behind me at the sas-plus desk.

Some norwegian hunters in front of me appeared to be checking in elephant guns/blunderbuss and large cooler boxes which took a while, wonder where they had been? Troll hunting in Scotland?

I asked at the desk and within about 30 seconds the bag was checked all the way through to bergen, as was I, without issue or argument.

Obviously I wanted to ask about the detailed business process flow, governance and technical integration of how and why this works...but it was 05:30am in the morning, so the internet will have to speculate as to whether I got special treatment for wearing my best and only shirt, or (more plausibly) it's just a normal thing for sas to sas transfers at oslo, even if the tickets were bought separately from sas.

Once off the plane at Gardermoen I went through immigration, followed the transfer signs, went through security scanner again,this then spits you out in the international terminal area airside (but before the bit you would go through passport control to get to the f gates for flights back to the Uk etc).

You then follow the signs to get to the domestic flight area, through double electronic gates for customs, scanning the boarding card both times but was not asked to review a picture of my checked luggage to confirm mine etc.

Hope the info helps someone else in the future.
​​​​​​​Thanks.