Back now from my trip and I can report that Roskilde is indeed a fine place.
Very little traffic, plenty of space, friendly people and very good value. Looking at my invoice, the parking fee for two nights was 92 DKR which is about £9. By UK standards the fuel (assuming you are leaving Denmark where there seems to be a reduced price) is a real bargain at about 66p per litre.
The AVGAS pump takes BP fuel cards. If you don't have one you'll need human intervention - the fuel office is just opposite the pumps.
The taxi to Roskilde station was about £15, exactly as MAW CPH says.
All-in-all, a good place to stop.
That Grønholt link looks interesting too. One to file away for next time.