It took a lot of searching but if you go here; http://www.hmso.gov.uk/si/si2004/20043258.htm and providing you have a legal dictionary to hand you will find the answer!
For those of you that cannot be bothered, I believe the answer is; If the vehicle exceeds 3.5 tonnes but not 7.5 tonnes and is fitted with a speed limiter then you are banned from the outside lane of a three or more lane motorway as of 01/01/2005. This will apply to all goods vehicle matching the weight descrpition first used on or after 01/01/2005 unless they are only used in the UK, in which case the magic date becomes; 01/01/2008. Vehicles first used on/after 01/10/2001 but before 01/01/2005 will need to have limiter fitted and working from 01/01/2006 (if used internationally) or 01/01/2007 (if only used domestically). Once limiter is fitted and working then you are not allowed in outside lane of 3 lane motorway, just the same as other HGVs’.