I'm going to be replacing my shower valve soon. I know the old one was
installed using plastic pipework, and this will need a bit of
reconfiguring for the new valve (basically, with the current valve there
are 90-deg elbow 15mm tap connectors on the feeds, and these will need
changing to straight ones.

I've never used plastic tube before, but AFAICR it's important to use
fittings which are compatible with the brand of tube, correct?
Particularly inserts within the pipe? I have no clue what brand mine is
(it's grey, 15mm) - maybe I'll find it printed on the pipe when I have
it fully exposed?

Advice appreciated...

Thanks
David