Not sure if the clutch plate can be stuck open, but I once had an incidence where the clutch slave cylinder piston was pushed to far out ant clamped. So the lever for the clutch was always in the "open" position and did not return.
Maybe something like this happened. This should be visible when you look at the clutch slave cylinder. If you get no further help here, maybe also check mgexp.com. ( Hope the webmaster does not beat me for this, but there are more magnette people that could help you)
kind regards
Simon

My Zb restoration is nearing completion but I am puzzled by the following issue The engine runs fine but the clutch which I recently filled with fluid and bled will operate by the gear stick through the gears with or without me pressing down the clutch pedal. In addition whatever gear is selected there is no feed to the transmission (the car is on blocks at the moment). The car had been stored over many years so I expected the clutch to be stuck on >What could cause this issue?