My Garmin had it at 6.38 miles - not that much longer than the expected 6.25 although long enough to make a time difference. It might be explained by dodging around the back markers in latter laps or just the GPS margin of error.
If I hadn't had the Garmin I'm not sure I'd have known when to finish as there seemed to be so many laps.
Action Duchenne is a great cause but to say that the start was "clearly marked" is pushing it a bit, we went off while dozens were still queuing for three loos (there were empty public ones around the corner) and the organisers' claim that all major runs lack baggage facilities on safety grounds is debatable at best (although they didn't respond to my e-mail questioning this). Luckily, my old joggers were so tatty there were still where I left them when I finished the race.
A good race and hard-working, well-meaning volunteers but parts of the organisation were absolutely shambolic - e.g the toilets at the start and especially the finish area. My wife was stuck outside Cardiff Castle, and was denied entry to the finish area I was stuck inside and spent almost as long waiting to get out as I did running 13 miles. The winner of the first prize couldn't make it to the first presentation ceremony - he'd got outside the castle and (initially) wasn't allowed back in.