Dave wrote:
I need to run concealed pipes from beneath the floor to wall-bracket
compression elbows for a wall mounted shower mixer, but can't decide the
best way to do it.
I'll need to cut a duct in the blockwork, is it best to mortar-in the
pipes or to use exterior ply over the duct (there'll be tiles over the
top)?

Me, I'd be tempted to go with the exterior ply option.
(I'd also keep aside the requisite number of tiles to replace ones if
access is needed to the pipe)

Is it best to use copper or barrier pipe? I'd be worried about the
mortar attacking the copper but presumably the plastic barrier pipe
doesn't need protecting(?)

Mortar will attack the copper - sooner or later.

Are there any problems using regular compression fittings on barrier pipe?

I'm pretty sure you need special fittings.

And a sharp hacksaw, or a dull axe to cut it with. (use a proper cutter, or
ensure that the edges are precisely square and burr-free.)