It's all very easy. All the answers are in that thread that Luddite linked to, so you don't have to find out the hard way like I did.
Seal kits come from Moto Rapido or Powerhouse.
Rip out the rubber boot to reveal the retaining ring, then warm the master a bit and prise the ring out carefully.
Make yourself a drift to replace the ring nice and square, the dims are in the thread and so is a description of my home made drift.
Don't welt the ring in with the drift, it's only used to push the ring down firmly and square with just a tap to finally locate it.
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