Investigative journalism, public interest and the law

A short article in the Sunday Telegraph on the weekend implicitly raised an interesting general question. It seems that reporters for the BBC's Panorama programme may have used faked doctors' referral letters in order to gain access to treatment at a fertility clinic. This may be contrary to the Forgery and Counterfeiting Act 1981. The programme's lawyers seem to have been aware and given the okay on the basis that the fakes were 'justified in the context of the undercover investigation'. So, quite when does the 'public interest' in the watchdog function of the Press justify lawbreaking during the investigative process?