Duncan,
Paul is almost correct, but you are also "Onto something" !
As I knew we would be coming back in the dark and I know the channel marker buoys in the run had been moved, I thought it would be a good idea ( incorrectly as it transpired ) to erase my older track out from Christchurch in order to get a good one to come back in on.
We then found out just how foggy it was and I realised it was a bad move. However, the saving grace was I had previously placed waypoints at each marker buoy position, so knew what way to steer and look. ( Unlike one boat we saw who was literally going around in circles with not a clue and I am sure nearly ran into SeaJay who was behind us )
Finding the buoys in the run was more difficult though as they had been moved. I knew we had to find them as we were heading out on HW and back in near LW and I needed a good track to follow back in.
As it turned out, it was a good job I drew a good track out as when we returned we found all the buoys ( bar one ) had been removed for the winter
Lesson learnt was : Think really carefully before removing your old tracks !