Turns out I was over thinking the whole process. ?
Just put all the discs in a separate folder called Phantasie II, call them D1, D2 etc. (so it is easy to see which is which on the LCD screen) and then use K2 to move between them at run time and K4 to mount. Simples!
Spent way too much time setting up multi-disc .LST files etc. on the C64 so I can play Wasteland.