I'd allow 15-20 minutes when you get to the stadium, the start points are spread out and the riding round and round to get to the right spot took me by surprise (I'd not done a sportive before, this may be normal for all I know.) It did feel like my start point was a long way round though (I was green I think, could have been yellow) I passed a lot of 'this colour start point here' markers before I got to mine.
I wasn't that happy with the approach either, I came in from the O2 (through the Blackwall tunnel) and the signposted route in on the other side took you through some pretty tight roads which were already completely blocked by cars, lorries and buses trying to get around the closures. I felt quite lucky that I commute through this stuff regularly because it wasn't the nicest of traffic and threading through on a cycle while the majority of the drivers were already pissed off at cycles for closing the roads in the first place wasn't the best. I didn't time it, but I wouldn't be surprised if it took me 30+ minutes to do the last 3 or 4 miles in and up to my staging area so I wouldn't leave things tight. I was aiming for my gate opening time and ended up caught short by a loo break as things moved on while I was in the portaloo, time was so tight. Then again people were mixing it up with start times a bit, so I wouldn't give up if you find yourself running a bit behind time because of traffic, punctures, whatever.
