My record is seven minutes - an unmanned platform late at night, and I wasn't using the amended diagram - my mistake, but no-body turned up after I had left, according to control looking at the CCTV, so I dodged a bullet. My mistake, I'll freely admit it.
However, I have had people accuse me of locking the doors early (30 seconds before departure time, as advertised), and people saying "My watch says it's XXXX", well tough, my watch is set by radio control (company issue), so that's what I go by.
As regards the Guard and SWT control "agreeing", that might be just agreeing with the customer as he is always right, and followed by "Now bugger off and annoy somebody else" muttered under their breath as they walked away.
I find that Trust is pretty accurate, and if trains do leave early, it is usually swiftly followed by a phone-call from control saying "What are you playing at", followed by paperwork and (if it is a common occurrence), interviews without coffee with both line managers and competence managers (who can ruin your whole week, never mind your day).
Edited to add: In certain circumstances, trains at Reading on the up, and Newport on the down can be sent off early, as there are so many trains leaving these stations for Paddington and Cardiff Central respectively that if a platform is needed, then the train will just be told to sod off and clear the platform for the next one.