Oh... Oh I see now...! I was doing it the wrong way round! *facepalm*
What I really need to do (which is what you've done Asmodean), is add the instance var to the family, then set it on the sprite that way instead, as it filters down to it (which I had no idea of).
It makes so much sense!
Thanks so much for that Asmodean, I owe you bigtime!