Stuart,
At that time of the day you should be able to get a move on and I suspect even with a stop you could be at the campsite before 9.30am.
If I were you I'd go straight to the site and once you've got the tents up etc go back and do the shopping. The site gets quite full by Friday afternoon but I think you'll be ok if you get there early, but as the last post said the best spots will be gone.
The main gate in case you were not sure is next to the museum, go out of the track and turn right, under the bridge, past the pub and the bus stop is just along the road.