Matthew Thomasson wrote:

I'm installing a grohe shower mixer.

The outlet that connects to the shower hose is an elbow which screws
into a half inch fitting mounted in the wall.

I'd intended using a half inch to 15mm elbow coupler (in the wall)
back to the mixer outlet.

But when screwed fully together the (15mm to half inch) coupler that
connects back to the shower outlet is left at an unusable angle.

The only way I can see it working is by not fully tightening the two
connections, but this would cause a leak ? What point am I missing
something ?

Any help appreciated.

Wrap the thread with PTFE tape, using sufficient tape so that the
fitting is fully tightened when pointing the desired direction.

Bob