If you're going for time then you have to go for a fast course. LN is not easy, I don't know the exact layout but there are lots of downhill sections and a few climbs. If the first half was downhill and the second uphill then you would expect a big positive split. If the first half was more difficult and you were aiming for 1h55 then probably too much effort. 5 minutes above your PB is not much. What are you friends HM PB ? Sadly there are faster runners out there and I hate them too !
I don't know Norwich but did you simulate the course during training runs. I ran hills thinking I was big and clever.... for a flat race, my legs were strong but not use to MP.
As Weedy said, other factors, 6 marathons is a lot as I've only done 2, however if one was too hot, another too cold, another too lumpy perhaps it's just a question of time before all the conditions fall into place.
OT - they weren't training runs either, over half were races, over mountains. Impressive stats from Dannirr.
Tinks - Is Econometrics, Fifty Shades for geeks, not to be confusing with MacPerky's favourite read 'Fifty Sheds'. Strange bedtime reading but I'm sure you'll sleep well. There are loads of UN reports but only recently they have been correlated together. Check out Hans Rosling speech on TED