As the valve is leaking I want to replace it with an alternative reliable make that will not create pluming problems.
The distance of the inlet pipes on the wall is 130 mm.

Demmie, October 2017

Look here:

https://www.showerdoc.com/sirrus-bramham-shower-spares

Specifically:

https://www.showerdoc.com/sirrus-bramham-shower-spares/question/dripping-shower-56747

and

https://www.showerdoc.com/sirrus-bramham-shower-spares/question/shower-still-drips-after-use-102997

There may be other relevant threads but you should be able to go through them for yourself.

Showerdoc may be able to advise on a suitable alternative if yu really want to go that way

sggs, October 2017