http://www.highwaycode.gov.uk/11.htm
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117: On a three-lane dual carriageway, you may use the middle lane or the right-hand lane to overtake but return to the middle and then the left-hand lane when it is safe.
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), mainly because the streach of M4 i travel every day (16-17 and back) is packed with lorrys going 56 and the odd granny stuck between them!... 