I have an old and badly corroded Lucas DPC XUD injection pump from a 306 1.9D 1997 fitted with the Anti-TWOC shield over the pump stop-solenoid.
Here's What I did to Remove it:
I centre-drilled into shear screws with a 2.0mm* 'jobber' drill bit to a depth of about 8mm.
Using a decent quality 1/4" hex-drive (Britool, after trying most, I found were unbreakable on corroded PSA door hinge pins) Tork 'T8'* bit and carefully hammered the bit into the drilled hole. The fit was really good and I didn't feel there was much chance of the Torx turning without the screw moving even though they were quite rusty having been there for twenty years.
So, no angle grinding or hack-sawing required
* These sizes are from memory so please make your own measurements and share below. the combination gives each of the points of the Torx bit about .15mm of 'bite'. I think.
Finally, my green 1.9D Xantia's MOT time comes tomorrow. I can take absolutely anything other than structural corrosion on the Fail Sheet
PS. also from the bedtime workshop, when replacing metal pipework on cars - maybe not for a lot of the pipework on Citroens with hydropneumatics - it's perfectly OK to Sensibly - with regards to safety - reroute them to make life easier and often results in a neater job for DIY.
Copper brake pipes, whilst easy to flare, can easily kink instead of bending. The 'Kunifer' alloy pipes are easier to do a really neat job, without kinking when doing curves. During my apprenticeship, a mechanic showed me how he rolled pipe lengths between two planks of wood. Useful for those doing restoration work to a concours standard or to impress the MOT person.
On my father-in-law's Rover 75 estate, I took a less circuitous route rather than dropping the fuel tank & other stuff. Just use common sense (and quality 'P' clips with rubber inserts). When a car is made, the Pipework is usually fitted BEFORE the vehicle's fuel tank and subframe are installed and that's the only reason they go all around the car in a most tortuous routing.