Just in the middle of this myself at the moment, and I can identify with the suffering. The manual covers all this in one page with gems such as "remove clip from cylinder" which is barely visible and completely inaccessible. The Alfa manual is not much use here.
I have removed the clutch pedal (not as in the manual, the pivot bolt withdraws as far as the fusebox which is not far enough -remove fasteners on fusebox and bend slightly).
Unbolt the accelerator pedal linkage and associated spring. You should now be able to get a 13mm socket onto the two nuts holding the master cylinder to the bulkhead. Lying flat on your back on the floor - the driver's seat is out by the way.
Pipework: Here the manual is taking the p***. The clip on the inside union is a joke, it's one of the Alfa clamp 'n clip types which should ping off with a suitably placed flatbladed screwdriver, however you won't be able to get a screwdriver near to it. It's way up high next to the bulkhead behind the pedals and ventilation plastics. Replacing the clip will be worse, so let's give up here and try plan B:-
The inside pipe connects to a metal pipe via a clamp 'n clip about six inches downline in the engine compartment, close to where the back pipe connector (the output pipe) needs to be removed. So I'm going to leave the inside one connected until the unit is out on the bench and undo it then.
As Kev described, access here is not in the realms of reality for humans, especially when stiff corroded pipe unions are involved. Here the manual says "remove the airbox" for access, which sounds delightfully quick and simple, however this seems to involve major engine dismantling with some access to bolts from below, so it's jack the car up time and send in the trained ferrets with the snapon kits.
My other plan (plan C) for changing the master cylinder involves an industrial angle grinder, lots of welding and replacement panels followed by a complete respray.
I'll let you know which is the best plan as it unfolds....