The problem was two,
1 the switch for selecting which drive unit to use was malfunctioning ( does anyone know where to buy a new one? Is it a standard switch? But when bypassing the switch the problem remained.
2 I changed the cource computer and then it worked again at least at anchor. So now I do not have any spare cource computer
Lesson learned; Install two totally independant Autopilot systems. Not a big issue to bypass as switch or changing cource computer at anchor, but at night in heavy seas not so fun at all. And an other thing problems look much smaller after a good nights sleep, a beer or two Difficult to achive when on passage and things need to be fixed asap.
Paul on SY Kerpa SM # 259 at anchor in Virgin Gorda