The Leaseholder's Handbook for your property states that "you need to seek authority from the council as housing landlord of your property" for "any change to the internal layout of your property":
http://www.crawley.gov.uk/pw/web/int121905
(See page 34).
So I remain confident that you need to apply (retrospectively) for permission to install a stud wall (and associated doors).
Even if I was wrong it wouldn't help you because the solicitor acting for the potential buyer would need to see
proof that no such permission was needed. (i.e. a letter from Crawley Borough Council stating as much). So you still need to apply for permission and see what CBC's response is.