I can't get the dynamo side cone to lock It has a 'hat' over it, and the lock not clamps to that, but that makes the hat turn, which turns the cone in. It's not like on hub gears which I understand
I see what you mean.
I've just had a look at mine and the dynamo side cone and lockring arrangement is different to that of the FG.
The "hat" seems to be a spacer with no means of gripping and turning the cone. The cone is meant to be tightened as far as possible onto the axle - until it runs out of thread. The magnet, spacer and locknut are then fitted. All adjustment is done from the other side.
I imagine your dynamo side locknut has been loosened at some point, and in the process the cone has been loosened a bit as well. If you loosen the cone on the other side you should be able to tighten the dynamo side lockring fully - hopefully it will take the cone to the end of the thread at the same time, but I don't suppose it is critical if it doesn't as long as the assembly is locked.Then you can readjust the cone on the other side.
I'm sure you know that the dynamo magnet and armature mustn't be separated. If you need to access the cone you can easily remove the whole dynamo assembly by removing the nuts that fasten the magnet to the hub shell. Remove the non-dynamo side cone etc and gently tap the axle through. The magnetic attraction between the parts will hold them together - just watch carefully to make sure the magnet doesn't jam inside the hub and become separated from the armature.
I got an exploded view from the Tony Hadland book but have just found this
which is clearer and has instructions as well!