I wrote:
I am fitting a thermostatic shower valve into a stud wall.

Sorry for responding so late on this.
Thanks for all the good replies by the way, it seems I was not alone in
this!!!

Having spoken to the kind chap at my local plumbers' merchants, I was
advised thus...

If using 90° Male 'Irons' (they are not compression fittings but male
threads that fit into the female threads of the shower valve. They no
longer seem to be supplied with any valves as manufacturers have
realised they can't account for the angle required by the feed pipes
for 'ALL' installations and it's cheaper to just not include them!)
using plenty of PTFE tape (or alternatives) is the answer.

Wrapping lots of it on, and perhaps creating a 'taper', near the
knuckle, by twisting the reel, as you wind it around, means the seal
will be watertight, even when the fitting is not tightened to its
extreme. Thus, the elbow can be rotated to suit the position of the
feed pipes.

Even so, this does take a bit of practice to get the right balance
between adjustability and watertightness, and it means you have to be
able to do a 'wet test' before sealing the valve up into the wall with
WBP ply, plasterboard, tiles etc which is not always convenient.

A better suggestion for this situation (and the one I used) is to
always use 'straight irons' (like compression straights but with no
olive or backnut on the shower-valve-side of the fitting).
This can be fully tightened, with PTFE, into the shower valve, without
any doubts about leaking.
To get the 90°, 45° or other degree angle pointing in the direction
of the feed pipes, simply use a 'Street' or 'Stem' copper fitting (of
the desired angle elbow) in either 15, 22 or 28mm. These are fittings
which have different diameters on either side i.e. one side 'holds'
22mm tube while the other 'is' 22mm. In my case, I used 22mm, 90°
street elbows... the 22mm side slotted into the straight-male-iron,
rotated to desired position and then compression fitting tightened,
locking it in place... then other side recieved small length of 22mm
tube soldered and linking valve to feed pipe locations. It's a bit hard
to explain without diagrams, but by using "Street" fittings, any
situation can be dealt with, without the doubt of leaking PTFE joints.

Hope this is useful to someone and thanks again for the replies.

rgds,
deano.