I've fitted a new shower mixer. As I couldn't find one with the exact
spacing for the original pipes, I went for one which has a pair of
cranked couplers on it, so you can adjust the spacing between hot and
cold inlets by up to 3 cm.

You screw on the couplers into the existing female threads in the wall,
adjust to correct spacing and then connect up the shower mixer which
has female threads.

The problem lies in the coupling attachment to the wall. It's screwed
in, but not fully tight. If it was then it wouldn't be adjusted
correctly and the mixer wouldn't fit.

I'm worried that with a power shower it will leak. What's the answer
here - is there some kind of cement I can use?

andyv wrote:
I've fitted a new shower mixer. As I couldn't find one with the exact
spacing for the original pipes, I went for one which has a pair of
cranked couplers on it, so you can adjust the spacing between hot and
cold inlets by up to 3 cm.

You screw on the couplers into the existing female threads in the wall,
adjust to correct spacing and then connect up the shower mixer which
has female threads.

The problem lies in the coupling attachment to the wall. It's screwed
in, but not fully tight. If it was then it wouldn't be adjusted
correctly and the mixer wouldn't fit.

I'm worried that with a power shower it will leak. What's the answer
here - is there some kind of cement I can use?

The answer is - depending on preference, a LOT of PTFE tape or hemp and
plumbers Mait (don't smoke the Mait ;-))

If you fill the threads upo enough, it won't leak even when not screwed
fully home.