Using 'I was just keeping up with them, so it was their fault' ain't gonna get you out of a conviction. You've got a speedo, you should use it. I always do, so I know exactly when I'm doing 110-120mph down the M40!
And FYI - if they pull you up and give you a ticket, they don't need video evidence otherwise how come speeding tickets have been around a lot longer than video cameras? The only time 'mechanical evidence' (camera, speed gun etc.) is required is if it was a policeman on his own (e.g. motorcyle). Otherwise, if two policeman are of the opinion that you were speeding, this is admissable as evidence.
Having said that, the fact that they didn't give you a ticket means you've got away with a friendly warning - lucky bastid. Still, I've been very sensible and my licence is now clean again......
I'm an old bastard - that's why Rach calls me Dad. We both like it...