I think there is ways and means around it Ade - not sure myself as I've only had this seal replaced at a workshop (we ended up breaking the special tool trying to get the pin out). I think the tool is to ensure the pin is centred correctly although it was some time ago and the mists of time have greyed it out a little
The link from B.J. on the workshop button above (see drivetrain repairs) shows how to replace the seal on a diff with a 'traditional' four bolt prop-shaft flange not the three bolt on the later Disco - this however should be fine for a how-to guide on doing your front seal.
Hopefully someone will know a work-around for the rear seal and will post soon...