In reply to Clears: It must be spring! Nope, sorry, there are currently o campsites on Portland. The Mermaid used to (slightly unofficially) rent out a bit of field but Lee and Dorian have moved on and the Mermaid is no more. I could't condone it myself, of course, but there are a few disused quarries that have been known to have a tent overnight (hint, behind the YMCA) or stay at the YMCA itself. Failing that, a bivvy bag near the Cuttings is also not unknown but PLEASE - in late, out early and clear up everything (even if it isn't yours!)
Don't forget that other people have to live there and climbers are not their own best friends at times!