From Oslo one line runs west to Bergen and gives access to the Hardangervidda or Skarvheimen areas e.g. at Finse. Geilo, a couple of stops further east is a town with facilities.
Another line heads north from Oslo to Trondheim, passing through Oslo Gardemoen airport, and then giving access at places like Otta, Dombas, or Oppdal to parks such as the Rondane, Dovrefjell or Trollheimen. There's been some fairly major disruption with bus replacements for extensive engineering work on the southern end of this line, for well over a year now, so check for current info if you're thinking of heading that way.
There are two main bus companies Norway Bus Express and Nettbuss Express
Buses will get you to some of the more western parks, the Jotunheimen or Tafjordfjell for example. They take several hours but are pretty comfortable. Norway Bus Express will certainly get you to Tyin for the southern end of the Jotunheimen. Nettbuss run through Otta and Stryn for access to the northern jotunheimen or southern Tafjordfjell - these services run on to towns on the fjords.
For some options you may need a local bus or else resort to a taxi to get to the starting point for your hike.
There's some more general info on this visitnorway page
Looks like a good trip, Graham. And quite surprising how reasonable the cost of getting there was.
The fact of it being so empty would certainly be appealing, and there's something about those wide-open views and big skies, but I suppose in reality I'm always likely to pick somewhere a bit more mountainous for my nordic ski touring. This year I went back to the Jotunheimen in Norway... I must say you've shamed me - I should try to get round to pulling together a few pics and a write-up.
I'm not sure about Gwern Gof Uchaf, but Gwern Gof Isaf a mile further east certainly offers day parking. Some time this year the price doubled from £1 to £2 (but still very reasonable), and you just pay by knocking on the back door at the farmhouse.
Only to a point.... if you load them up with heavy, bulky stuff, then yes, absolutely. But as a convenient place for something low bulk and weight - maybe a compass, or a map printout folded into an A6 Aloksak, or even a mobile phone with gps/mapping - then a thigh pocket works just fine, even on close-fitting, stretchy trousers, and personally I find one pretty useful.
Perhaps it's just terminology - the article says thigh pocket and you've translated that to cargo pocket. I'd say the former isn't necessarily the latter.
It's not really a shame - it's a brilliant place to go in the winter. But I did really enjoy my one summer( -ish, it was -3c and snowed the first couple of nights!!) backpacking trip (to Trollheimen) and would like to do more. The only thing preventing it has been insufficient holiday time and competition from other great places. With Norway getting an almost-annual winter trip, areas like the Pyrenees, Corsica, Picos, Dolomites, Scotland, cycle touring, etc. have tended to eat up the summer time... so far.