At the roundabout just before the motorway - J10a I think - I got in the middle lane to go straight on. Then as I got on the roundabout saw the sign saying "M1 North", thought "oops" and carried on round... ending up coming off the roundabout as if I'd turned left.
As I came off the roundabout I thought "silly me, I do want to go North", so turned around at the first junction and came straight back up.