Galaxy Aqua 1000 Knob Pack (white) SG08089
There were instructions to re-orientate the 'd' position on the shower mechanism to correct the problem, but that seems an awful lot of bother for what is a push-fit knob.
Having said that, the fit was perfect and all the spacers were included, so it was an easy replacement job if the orientation doesn't bother you.
Ian Mansell