A single white line on the edge of the carriageway is used to emphasise a centre hazard line. Centre hazard lines are either 4and 2 (4m line, 2m gap) or 6 and 3. They are used where it is OK to overtake, but you should be aware of a hazard eg slight bend, approaching junction. Where there is a normal centre line (2 and 7), there is normally a broken line on the edge. Having said that, edge lines are normally only used where there is no kerb. Sometimes this comes about because the road has been upgraded to have kerbs and when it gets re white lined, the gang just go over all the old lines.