Interesting to see post above mention conditions. Not seen anyone mention it.
No idea why anyone cares what I run, but I'll play along......
I actually only looked at the course map for the first time maybe last week, staggered to find most of the course goes east to west, so doesn't take a genius to work out why London times can be variable, even running in groups.
If it's an easterly wind, I'll probably just take the ski slope looking downhill into a headwind, take the 9+ miles tailwind, set a massive half PB and jog the rest in and you'll all be glad to never hear of sirpoc again.