I may be mistaken, but I'm sure I've read on here that Drayton TRV4s
don't care about the direction of flow of water - i.e. they can be
fitted on flow or return, and don't care.

I've bought one to have a look, and this isn't what it says at all.
The one I bought said "reverse flow" - though all the ones I've seen
on-line seem to say this. You _can_ fit it to flow or return, but you
need to know which is which because on flow it needs to be mounted
vertically, and on return it needs to be mounted horizontally.

This makes sense. So, is there a version of the TRV4 which is truly
agnostic to the direction of flow? If not, can I fit these in the
wrong way without breaking them, and swap them around if they play up?
Because there are parts of the system where I think it would be far
easier and quicker to do this than to find out which is flow and
return before fitting them!

(system is completely in pieces now with no rads fitted, and some pipe
runs missing).

Cheers,
David.