I am very confused..
Ok.. So I have replace the gear cable. Problem still shows itself. I test by going astern on the lever and turning the prop against engine compression by hand. I can hear the dog clutch clicking away and can see the gear shaft popping up and down. So this at first guess sounds bad. Then I separate the nuts holding the drive shafts together, remove the locking bar which pulls the locking mechanism down when astern (which I only recently obtained as it was missing when I bought the boat). I manually engage astern by pushing down on the lower part of the gear shaft and manually turn the prop over - hey presto she stays in gear.
It seems to me that the spring tension on the locking bar thing is pulling her out of gear... Any thoughts what to do here?