Problem with clutch slave cylinder replaced under warranty 3 times. (autoexpress.co.uk)
The Astra H has a concentric clutch slave cylinder which means the gearbox has to be removed to replace it, 5 to 6 hour job. (mechanics.stackexchange.com)
Miatas of this age are known to develop leaks clutch slave cylinder causing the inability to disengage the clutch to shift. (gearpatrol.com)