I've now found out why you cannot access the nut on top of the Swivel Pin from inside the boat. It's because the Transom is in the way.
When the hole is cut in the Transom for the Transom Shield (inner housing) to be bolted to the Gimbal Housing (outer housing) it has to be a certain size, but it's not big enough so that the Swivel Pin nut is accessible from inside the boat. You would think you could cut a small bit out to get to it , but not so sadly.
Taking off the leg and the bell housing reveals the Gimbal Ring attached to the Gimbal Housing. The Swivel pin is the top pin to which the Gimbal Ring is attached to the Gimbal Housing. It's nut attaching the Swivel Pin is inside the top of the Gimbal Housing so getting at it means cutting a 1 1/8" hole either side of the top of the Gimbal Housing.
Some say you have to undo the nut by using a screw driver on the edge of the nut flanges with gently encouragement from a hammer, but others say yo can just get a spanner in there.
Either way, once you have cut the holes, have access to the Swivel Pin and managed to undo the nut, the pin will drop out.
Employing someone to do it, for me would be ringing round the Marine Engineers in your area and asking them if they had done one previously. If no, move on to the next one. Some of them do invoice more than the actual hours worked, which includes chatting to the wife or taking other bookings on the phone and then of course there is the working out what to do next if they haven't done this sort of thing previously.
There are some really good caring engineers out there though but it's finding them and they are usually very busy.