like if I removed the cylinder, see below - then the pipes (of which there are five or more) would have to be chased nicely into the wall
Not necessarily - if you are planning to set the whole thing 200mm shallower, you could box in the pipes running up the back wall, which even for the 22mm pipes would only mean, say 55mm max. projection, allowing for the projection of 22mm pipe clips and plaster board itself as well as a thin skim of plaster.
It would mean you'd still save 145mm overall depth of the whole thing.
I guess I could just have a 90mm box (that's how much I need for where the pipes are) against that wall, instead of a 650mm "box" to also contain the hot water cylinder.
Still, it would be a tremendous amount of work to move the thing, not really worth it IMO, getting it set up in the attic. The storage space "lost" by not building this cupboard will just have to be made up elsewhere, as well, so I wouldn't actually save the whole footprint of the cupboard.