Well, as I'm a cornish boy, I can tell you a lot about this place. ;)
As a young engineer back in '98/'99, I was involved with the original preliminary setting-out of the place. Bulk earthworks, drainage, the visitors center, biodomes etc etc, even when the pit was still being used by the china clays company.
I can tell you where the mineshafts, wells, drainage channels are
Seriously though, I went there 2 years back to see how it turned out. Intresting place.
As for places to stay, Carlyon bay is really the only place to stay in St Austell.
Although to be honest, I wouldn't stay there ;)
Much better off in Truro (where i'm from, only moved from there to London in the search of money 3 years ago), Newquay or Falmouth. Newquay and Falmouth have a greater choice of hotels, but Truro is the "nicer" place to stay, Newquay can get a bit rough at night, although this time of year it isn't too bad. I'd avoid the place like the plague in the summer though.
Feel free to ask any questions :) Spent 23 years of my life growing up there :D