Were you on the first shuttle bus back to town? I was standing near some people in the queue who were saying exactly the same.
Re the measurement I know GPS isn't super accurate for course measurement etc etc, but I really didn't deviate much at all and had a clear run without much overtaking etc and do more than a dozen races a year usually so am pretty used to my garmin reading about right...and mine was 13.4 too. Sorry you missed out on the PB's
. Lucky for me I'm doing London next week so was going for a steady run and wasn't really worried about time, so it didn't actually bother me at all, but thought I'd respond anyway to say hello 
Otherwise a great race though I thought. The organisation at the start especially seemed loads better than previous years (well done the organisers!!) - except perhaps the cunningly hidden ladies baggage truck almost completely eluded me! (The compulsory gender split of the bags for transport confused me as after reading the race guide leaflet had planned to leave my bag on the London road truck) Admittedly it was a big truck and hard to miss once you turned all the necessary corners round the back of the stadium
, but a marshall told me it was in the car park and it took me 15 mins to actually find! I'll know for next year though..
Well done and thanks to the organisers!