Blimey Jim.
I moved this thread to the collectors section as I thought folks here would be all over it like a rash.
The leather MK2 seal is held in place by a plastic ring which is stretched over the front cone of the piston on assembly, to get this off you normally destroy it.
The new leather seal normally comes with a new plastic retaining ring --- hot water is your friend.
The whole of the O ring adaptor has to be stretched over the cone to fit it --- I think Chambers insist on you sending them your piston for fitment as it's a bit of a bugger.
All the best Mick