Mmm food for thought there.
Regarding the seal theory, the worse slippage was experienced on the first heavier use of the throttle following a longer journey and a 4 hour cool down.
In my mind i can make sense of that. Longer journey,engine block at hottest, oil thinnest and time for a leak to allow an amount of oil out to contaminate the clutch plat/ flywheel before setting off again easy at first spreading the oil and then slipping at first use of larger amount of torque?
I have also been thinking about the release bearing sticking on the shaft or master cylinder sticking. The clutch doesn't need a great deal of travel to induce slippage.
When I was trying to solve the dry feeling clutch pedal operation i noticed the action of the master cylinder to be a bit dodgy. It was notchy in it's movement, aha I thought that relates to the fell at the pedal.. I lubed it and things visibly improved but it didn't improve the dry creaky feel at the pedal. It's action looked full enough in both directions. I did notice the design seemed to be asking a straight in and out action of the slave to cope with operating the clutch arm in an arc which I put down to poor design, seen that before. This action making the operating arm of the slave bind on the internal face of the cylinder.
But with this being a release issue if it is indeed a mechanical fault i had counted the slave out as the problem.
Tis worth a bit of thought and head work before I get my hands dirty as the car drives perfectly most of the time. To the point where I am just thinking it's fine when it slips again
Thanks very much for your input, it is so useful to talk about stuff.