Having stayed there last year I can recommend the Holiday Inn Express on Old Street. There is an underground car park just down the road at Finsbury Square and its a nice 20 min ride to the Olympic Park, mainly along the A11, which is well sign posted from the car park. For anyone in the party who is not riding you are not too far away from the London sites. You also don't have to worry too much about road closures, we were able to get in and away without any fuss on the Saturday and the Sunday last year. I'll be using it again this year.