It's a pretty simple answer that seems to be down to MacOS thinking for itself.

SublimeText saves the file as makefile.cpp, even when I tell it not to append .cpp

Then, when I go and rename the file (removing the .cpp), MacOS doesn't delete the .cpp, it just hides the extension!

So...

I have to 'Get Info', deselect 'hide extensions', then delete the extension there... every time I save the makefile; which kinda defeats the object.

I'll have a poke around and see if I can stop MacOS and SublimeText conspiring against me.