Originally Posted by hotnail

For the record I almost solved the problem: I left a brick on the clutch pedal for couple of hours.
That made going into the 1st almost impossible - the master cylinder level was around "min", topping up fluid level returned back the first to pretty much like it is used to be.
There seem to be no traces of liquid around the master cylinder, so it could be that some air got into the system during the recent track day (did it boil ?)

I should do proper bleeding, it is somewhat difficult at my place...

Thanks to all the suggestions - looks like it is so far a £10 solution (a bottle of DOT4)