EDIT: Just to put this in to perspective, I know that most are seeing where I'm coming from, but what some of you are saying is...
One of you guys are travelling down the motorway at 70mph in lane one, I can crawl up the side of you in lane two so that I have my back wheels level with your bonnet, I then slow down, which in turn means that you have to slow down otherwise you have undertaken me. 
If that's the case, I'm going to have immense fun on my way back down the M6.

And to add to that, nobody has changed lanes to undertake, which is naughty, I just stayed where I was, in lane one. To exaggerate my example, hypothetically, a car in lane one is travelling at 60mph, the car in lane two overtakes him at 62mph, so is crawling past. A car in lane three overtakes at 65mph but just as he is drawing past the vehicle in lane two the lane three driver slows down.
Would you automatically expect the vehicles in lane one and two to ease off, so that they couldn't be accused of undertaking.........no, thought not.

That's what kept happening to me. No lane changes, just people slowing down for no reason.
