Citroen DS 23 Petrol Tank Removal

I've completely stripped down my DS 23 ready for welding but can't
work out how to detach the petrol tank from the fuel line. So far as
I can see there is a continuous polyethylene pipe running tightly
through a metal tube into the tank and this is protected for a foot or
so by a thick rubber outer tube. I have the Citroen manuals but no
mention of this little detail can I find. One flick of a sharp knife
would do the job but I'd rather do it the proper way.

Thanks for any advice.

JD