Charles Spencer was shown the forged bank statements by Bashir, and was convinced enough to arrange a meeting with Diana. He took copious notes during the meeting, of all of the things that Bashir said - Charles' affair with Tiggy/HM with heart condition/spy watch for William. There were 14/15 accusations in all. Apparently Spencer was also told that PC hated his then wife and wanted her dead (PC had only met once, at their wedding), at which point Spencer realised Bashir was a fantasist. The meeting ended, and Charles Spencer apologised to Diana for wasting her time.
She was sufficiently guillible to be taken in by the lies, which reinforced her paranoia, and kept in touch directly with MB, without her brother's knowledge, culminating in the documentary.
Charles Spencer tried to raise the issue at the time, but couldn't prove that he had been shown forgeries. The BBC conducted an internal investigation which, surprise surprise, found that there were only minor failings, and that Diana hadn't made her decision to proceed based upon the forgeries.
Only recently has the 'forger' come forward, which is why Charles Spencer and the BBC have resurrected the issue.