try hendra mynach site it is close to the promanade with a 10 min walk into town.we have used it a couple of times, you will find all details on the search facility on this site.
There are a number of sites in the area most of which you will find listed, and usually reviewed, in Camp Site Search. Apart from Hendre Mynach, a commercial site at the northern end of the promenade, you would pass several others between there and Harlech, from sprawling Holiday Parks to farm-based Certificated Sites. Also there are are couple of sites on the opposite side of the Mawddach estuary in Fairbourne, one of which I've not found listed and have forgotten the name of, a fairly basic but quite adequate site next to the station. Also in the Harlech direction there is of course "Europe's Largest Campsite", Shell Island (with its own official and unofficial websites if you want more information).
Went to Hendre Maynach for the Motorcross,A nice site not much on it but clean and a 20 miniute walk to the town.Not stayed at Nantcol but parked and walked about for a few visits now,Recently had a toilet and shower block installed, as it was proper wild before(portal loos and the stream to wash in).WE stay here in the spring to summer months and love it.Bit furthger out of Barmouth but right next to a beautiful beach.The web page is down and i can only asssume they doing it up, saying that they been doing a lot up on this site for the last 3 years.
Didn't think much to the staff at Hendre Mynach so a change over owners may be good,shame about the trains shaking you out of bed if you are on the railway side of the site.