I just had the exact same problem as the OP while replacing my clutch slave cylinder. Removal of the spring clips on the cylinder was nothing compared to trying to disconnect the rubber hose from the steel hydraulic line. I thought that I would share my experiences and what I did as a solution.
My problem started when I tried to remove the flare nut that connects the steel hydraulic line to the rubber hose (see diagram below). The rubber hose has male flare end that extends about an inch past a 17-mm hex-shaped portion. That hex portion is held in place (supposedly) by a C-shape bracket that is welded to a vehicle cross-member below the bell housing.
As I tried turning the flare nut, two things happened: (1) the hex portion of the rubber hose end began twisting in the bracket; and (2) the steel line began to twist with the flare nut. Neither of these things is good.
Because I was replacing the hose (and didn't care about damaging the old one), I clamped it to the cross member to secure it, since the bracket prevents you from being able to put a wrench on the hex portion. But that didn't do anything for the steel line twisting.
I didn't want to replace the steel line but nothing worked to separate the line from the flare nut. I tried PB Blaster and liquid wrench, but 26 years and 166,000 miles had taken their toll. So I made the decision to replace the line, without really knowing how difficult it would be to find a replacement. I cut the metal line, and then was able to remove the flare nut from the hose and remove all the old parts from the car. I disconnected the steel line from the master cylinder.
Rather than looking for a a 25-year-old replacement line, I set out for a local solution. As you know, you can buy pre-flared brake lines in all sizes and lengths at any auto-parts store, but none of them (at least that I found) has the female flare nut on them that is needed to connect to the Alfa rubber hose. I was going to have to be more creative.
Eventually, I decided to buy the right diameter and length of steel line, cut it with an appropriate tubing cutter, and replace the fittings on the line with the old fittings (including the flare nut) from the old line. I would then need to put an appropriate flare on the end of the line that I cut.
Taking the old line with me, I purchased a 1/4 in x 40 in tube for $6 at a local auto parts store. I looked at both the 1/4 in and the 6 mm tubes, and am convinced that the old one was 1/4 in. (Maybe a PO had replaced it at some point but given that it uses flare fittings, the small difference between 1/4 in and 6 mm wouldn't matter). I cut about 6 inches off one end so that I could replace the fittings. (At this point, the line was longer than needed, but I wanted to bend it to shape starting at the end connected to the master cylinder and then cut the other end to length when I got it aligned properly).
Next was to get the tools to bend the line to the right shape and to create the flare. I went to Autozone and was able to get a tube bender and flare tool at no cost through the loaner tool program. (The flare should be a double flare, which I had never done before, so I practiced on the 6 inch piece I cut off until I was comfortable with the process.)
Using the old line as a guide, I used the tubing bender to bend the new line to the proper shape. I attached it loosely to the MC and then marked the line at the proper length for attachment to the hose. I removed it from the car, cut the line at that position, and used the flare tool to create the proper double flare.
I reinstalled the line at the MC, connected the hose to the line using the old flare nut, and tightened the retention nut on the hose at the bracket. I bled the clutch, and it is working great with no leaks. Sorry I don't have any pictures, but this is not a difficult procedure if you run into the same problems I and the OP had with disconnecting the hose from the metal line.