Well if the garage can't determine if the master cylinder is good or not without removing the gearbox I wouldn't be using them.
A simple refill and bleed of the circuit and then disconnecting the quick release connector and pressing the clutch pedal will tell you if the master cylinder has failed or not - the pedal should be rock hard.
Another quick test for determining if the slave cylinder has failed is looking for fluid coming out under the gearbox where it bolts up to the engine.
Don't let them replace the DMF unless they have measured the radial and axial play and it is beyond the wear limits. They usually don't need replacing - my DMF is original at 404k miles.
If you aren't sure about the garage get them to refill and bleed the system and try contacting Phil T4 on here - if the garage he works at will do it then you know you will get a good job, and well worth a trip to Olney.