In reply to Nigel R Lewis:
Gardermoen airport is not a large airport, and this should go fine.
Two possible problems:
Delayed airplane... Not much you do about that.
Baggage reclaim... Normally done in appr. 10 minutes. I assume you can't travel with hand baggage only as you will need lots of winter gear at Haugastøl at winter. If you carry some kind of special baggage like skis, these are delivered at another place than the conveyor belt, and will normally take longer time to be delivered.
Once you have got your baggage, walk out through customs and head immediatly to right, 150 meters and take the escalator down one floor and you are on the platform for the flytoget.
Easiest way to pay is to use your credit card when entering the escalator. This is unassisted service, just mark your destination as Oslo S. Will cost you appr 20 £.
From the baggage reclaim area to the the train you should not use more than 5 minutes.
At Oslo Sentralstation it is possible to get from the flytoget to the train to Haugastøl in 2-3 minutes, provided you know the platform number for this train. All lines/platform is connected with an underground corridor directly from the platform. Down in the corridor there is signs with information of which trains that are the next to leave from each platform. Your train will have the destination Bergen.
The platform for trains to Bergen will usually be at low number, 3 to 7, but it can be changed due to trafic issues.
If you take the standard exit from the platform to the main entrance hall in Oslo S, there is a large information board diaplaying all in- and outgoing trains with departure time and platform number. Look for the train to Bergen.
Even with this small detour to the entrance hall, you should manage the shift from flytoget to the Haugastøl train in 5-6 minutes.
Tickets to the train can be bought on board the train, but then with an extra fee. Otherwise this can be bought at the entrace hall or online at
https://www.nsb.no/en/