Steve,
The holiday camp site is in Axbridge, however you should be able to get day tickets from Veals in Bristol as well, cost is about a fiver. A word of warning though, because it's a huge concrete bowl if the wind picks up you can find the waves lapping around your feet & it can be quite difficult to fish.
As for South Cerney, there are two lakes in the main Cotswold water park which can be day ticket fished. For Tench I would go to the one called Top Pit which is the smaller of the two (not that either lake could be called small!). Ashton Keynes angling also hold the rights to another lake called Cokes pit, apparently this is very good for Tench but is closed until 16th June. I haven't fished it myself but I intend to.
Regards
Rich