I've had the exact same problem, only I've twice been stranded on the road when the clutch pedal went straight to the floor, with no pressure, and made it impossible to change gear; the first time it happened I was just entering a steepish incline on a viaduct and wanted to change down to third. Fortunately there was enough impetus to get me half way up the hill and park off road. The clutch was replaced completely ( and the clutch master cylinder replaced, or so I was told) and for three months or so the car behavioured normally - then two days ago the clutch failed again in an underground carpark and it took a lot of stopping and starting to get the car out and then to negotiate two major roundabouts, all in second gear. But then the car recovered and we drove home (30km) without further problems. The following day I took the car to my usual mechanic who, over the course of two days, was unable to locate any problem at all and the car seem to be back to "normal" again. However the following afternoon when I had to go pick up a brush cutter it repeated the scenario with a loss of clutch pressure and impossibility of getting it into gear. I'm now thinking of taking it along to the local Opel dealer/motor repair shop to ask if they've knowledge and experience of the problem..Maybe it's just a sign that it's time to send the car to the scrap dealers - it drives well (when it drives!) and still has plenty of power, but it's done 240,000km. Perhaps I shouldn't expect to get any more km out of it.
The car has done well at that mileage and doesn't owe you anything ( trade saying ) Haven't heard of a definitive answer to the problem and still hearing of many with lower mileage having the same fault .