Thanks I was thinking of applying a small amount of heat with it intact to try and shift it
The hard lines are unlikely to be very dear. You might have access to prefabricated standard length lines that can be bent into a suitable shape.
Or, borrow a flaring kit and make a new one yourself. A good spread of options for this situation.
You will be able to apply plenty of heat without fear of damaging the seals. The cylinder end seals are covered by the backplate and the internal seals are insulated by the hydraulic oil. The elastomer should be capable of withstanding 200*C anyway.
When hot try tightening a little before backing off.
A mole grips may succeed if you have the access.
The hydraulic seal depends on a tapered fitting so the actual tightening force doesn't need to be huge so even if the nut is rounded you will able to do it back up correctly.
If you try and cut the pipe and put a joint in you will need a very good quality flaring tool which is expensive.
A cheap flaring tool is only suitable for copper pipe and as such you would need to replace the whole pipe run.
Have a try undoing it and see how you get on.
sometimes corrosion sets in between the pipe and nut a small amount of heat will free it off.
Wear goggles if using heat on pipes and unions, or working on overhead pipes with hot brake fluid that can squirt out into your eyes .