Having got throughly confused by the strong opinions on this thread, I thought I'd do as flowers suggested and google it. This led me to the ask the police website, a useful page where you can ask police offers these sorts of questions.
This is what I asked:
I am driving at 70mph on a clear motorway in the left hand lane and I come across a driver going slower than me in the middle lane. How should I overtake this driver? Is it correct to move to the right two lanes, overtake and then pull back into the left lane. Or should I continue on in the left lane, which will mean I pass the driver on the left?
And this is the response:
Thank you for your email.
Rule 268 of the Highway Code states:
Do not overtake on the left or move to a lane on your left to overtake. In congested conditions, where adjacent lanes of traffic are moving at similar speeds, traffic in left-hand lanes may sometimes be moving faster than traffic to the right. In these conditions you may keep up with the traffic in your lane even if this means passing traffic in the lane to your right. Do not weave in and out of lanes to overtake.
It is an offence to 'undertake' i.e. overtake a vehicle on the left. If you were to overtake a vehicle on the left in the circumstances you describe in your email, you could be prosecuted for careless driving.
Whilst drivers who hog the middle lane are annoying, you should overtake in the outside lane when it's safe to do so and then return to the left hand lane.
We hope this information is of use.
Regards,
PNLD
(CS)
I hope this helps to clear things up for people who have been as confused as me by this thread.