Access is pretty terrible at both ends, so I decided the only way I was going to get the car mobile again was to replace just the rusty section. In theory, this should be straightforward; cut the affected section out, flare the pipe on each side and use a pair of in-line connectors to splice in a new section.
In practice, this gave me a problem because I only have a bench-mounted flaring tool, which I bought from Pleiades about 25 years ago.
I know people have managed to use this type of tool on the car, but being cam-operated, it's really not designed for that type of application.
I couldn't find anything else at a price I was prepared to pay, so I decided to try making my own, using the punches and dies from the Pleiades tool I already have.
Hopefully this will inspire someone else to do the same and also spawn suggestions for improvement.
I found a nice chunky piece of U channel in my reclaimed metal stock, so that became the body, and I made the other parts to fit around that.
The new tool had to be screw-operated, and flaring steel pipe needs quite a bit of force, so I threaded a piece of bar M20 x 1.0 pitch
This is the kit of parts about 2/3 of the way through the build
This is the main part of the tool, just about complete. (The punch and dies in the picture are DIN style, not Citroen)
The first attempt at flaring a piece of steel pipe was a bit disappointing
I'd diverged from my original plan a little, deciding to use the later style of screw-in punches, as this would make it easier to incorporate an extraction mechanism.
What I didn't realise until late in the process is that the new punch is narrower than the original one and a very loose fit in the dies, hence the lop-sided flare.
(Original Pleiades punch on the left, new style on the right)
So, I made a sleeve to bring the diameter back to the same as the original punch
This is a close up of the punch holder, showing how it fits over the compression screw. The idea is that the punch is pulled back as the screw is unwound.
The completed tool, with a quick coat of spray paint
In use today
The result
At the risk of sounding smug, I was pretty chuffed with the result
Now I know where to come if I get rusty pipes!! 
