Vauxhall Meriva intermittent clutch problem

New clutch fitted last summer on 2010 Meriva 1.4 Turbo, running perfectly. Three short journeys last Thursday were trouble free, then final journey home the clutch pedal dropped to the floor and was unresponsive! Engine stalled and I had to restart the car, pedal now seemed fine. Difficult to engage first gear initially, but then moved off okay. Few mins later the same thing happened. Restarted and pulled into layby. Pedal seemed fine but would then become very soft and drop to the floor. Wait a second and fine again! Called out breakdown service, they moved it onto and off their vehicle without problem and garage can't fine fault, they've even taken it for a 10 mile run! Any ideas?