Mr ellevie -
There is no plastic padding or other 2-component filler product that can/will withstand the Citroen hydraulic system pressure of some 170bar (~2500psi) max. Also no type of rubber, plastic or polyethene hosing will match the 170bar recommandation - as these pipings are rated at max 10bar - some at 20bar. Your "great" repair is a ticking safety bomb - to say the least
It may be perfectly suitable for central heating with a max pressure of 2bar (safety valve opens at 2.5bar) - or the cooling sytem in a car with max 1bar (cap valve opens at 1.5bar).
The silver soldering technique is a proven and lasting repair - provided you have some training in doing it correctly - and can spot your own no-go'es.
There are 3 sizes found in the BX hydraulic piping : 3.5mm, 4.5mm, 6.35mm. I found out by coincedence that the larger size always fits as a bushing for the next smaller size.
If you live in the UK - there is really no excuse not to go get a new pipe. Its defo not worth the hassle and hazards repairing an old pipe. Emergency repairs done in remote corners of the planet is a totally different story. But people out there always seems to know whats good and whats dangerous