Well, it's done, now lets see if there are any more leaks, fingers crossed there are not !

For anyone who follows and might want to change this switch, the following information will be of value.

1). You will hardly lose any fluid at all, just a few drops
2). There is a plunger and rod inside the transmission that will pull out with the switch, the rod on the end will 'wobble' and is only loosely attached with a plastic collar. This must be re-inserted into the transmission, and will need to be 'wiggled' to fit back into the transmission.
3). The knurled collar holding the electrical connector will probably be stuck, make sure as you undo it, you don't tear the rubber boot over the connections.
4). The switch itself is torqued in pretty tightly, you will have to swing on it a bit to remove the old one.

Other than these points, it's a piece of cake ;-)

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1989 W124 300E
1991 R129 500SL