Here's an article written by a biker who is also a solicitor. It's based on the advice of Jon Hullis, a motoring offence solicitor and Len Tempest a barrister of long standing.
It follows on from the case of Ken Clark who was leading a ride-out and was doing around 85mph in a 60 limit when the two bikes following him were trying to catch up and doing over 100mph and the implication was that, as he was lead rider, his speed was an aggravating factor in the actions of the other two bikers' speeding.
Despite the way this was reported in the media, it seems that things aren't as bad as has been made out:
http://www.bgtbikersolicitors.co.uk/case_law.html#group_ride(PS Mods: I'd suggest making this a sticky because it's very useful info)