Really interesting reading everyone's thoughts/tips in terms of their split times versus how they trained and how they could improve.
I'll fully admit to not really listening to the "LSR should be slower than MP" advice and just doing my own thing. Looking at my training log, I was consistently faster than my marathon pace on all my LSR's. I just wasn't really sure what time I was looking at since it was my first one, so I just ran to how I felt. All of my LRS's had a pretty even pace throughout and felt comfortable (apart from my last awful 17 miler), so I figured I was OK. 2 weeks before London, I knocked 8 mins off my HM PB with 1h46.
My half way split was 02:01:23, so pretty much a perfect split as I finished in 4:03:32. It's interesting to see when I look at my last few miles, I knocked out a 8:39MM at mile 20 before smacking into the wall at mile 22. Even so, my pace only dropped to 9:20MM for 1 mile before I managed a 9:03MM and a 8:44MM for miles 24 & 25. I felt horrendous at the time. Almost like I wasn't moving!! Can't believe I maintained the pace. Really odd. I think at a different, less crowded marathon I could possibly have bagged sub4 though. I guess I'll find out when I run Leicester Marathon in October... eek!!
Anyway, for those who are disappointed with their times. Every single person in this thread has commented on how danm busy the route was. I wasted a lot of energy getting stressed and running around slower runners & walkers. When you're aiming for a specific time (I had set my expectations of my first marathon a little high) then you're going to waste both mental and physical energy trying to keep the pace up despite the crowds. Also, the support was defening at times and I got quite overwhelmed at a few points by the amount of people cheering. That's got to take something out of you too. Plus... well.. it's London innit. All that energy wasted on the excitement of the occassion when you should be running.
In short, stuff it. There will be other, quieter marathons where you can achieve your full potential. The main thing is that we took part and WE DID IT. Well done everyone 
Ooops, sorry for the ramble