Mira 415 shower runs hot and cold whilst in use?

Hi, my Mira 415 shower runs hot and cold whilst in use.
I have replaced the cartridge thinking this was the issue, however it still runs hot and cold whilst in use?
Could it be that it is running with a combination boiler ?
This is the very early type pre 1990 and has been very good, but because its in an internal wall that is tiled I am reluctant to replace unless absolutely necessary.

Any ideas please?