The Alpha bus does Thorn, Salcombe Regis, Oakdown and Kings Down Tail.
I've stayed at Salcombe Regis, a but plusher than King's Down Tail but much more crowded and all the site at King's Down Tail is flat.
At Salcombe Regis my pitch was sloping (though not as bad as at the official site where every time I awoke I was like a foetus at the bottom end of the tent).
I've used KDT lots of years. Making a late decision to come I came down Monday. Lots of folks go home Sunday night so while you might struggle to get a last minute pitch on Friday you'll probably be OK Sunday night.