If you can find a copy of it there is a nice piece on P5s and P5Bs in Classics magazine.
I don't beleive their research has been all that thorough. Apparantly you can pick up a P5B coupe in concourse condition for £4500 - if anyone knows where this is, can they let me know! The prices of the other models are likewise understated. Other than that a very good article, goes through all the things to look out for when buying one, and is very positive.