When you gain access to your concentric slave cylinder then you'll see that is leaking, not by much, but it doesn't take much to lose all pressure.
It's a really dumb idea to have the slave cylinder inside the gearbox, many cars are the same now including the new Mini. With an external slave cylinder then it's £50 for parts and 30mins to change. With the concentric slave then it's £100 for parts with 4 hours work and £250 garage bill for access alone