Thanks for the tips!
I actually just sent a mail to that Superior Software site.
Also, the name "Andrew Catling" appears on the main screen in this demo.. it seems like he did the art?
I tried searching for his name and found some computer game related names but from what I gathered, they hadn't been in the business for that long so it seems I haven't found the correct Andrew.
Sent a mail to one of them but I doubt it's the right person.
I'm quite bad at finding stuff on the net.
EDIT -
Hmmmmm.. have a look at this - the backcover for the box -
http://acorn.revivalteam.de/Pictures...tninjaback.jpg
Has some very nice screens of other levels than the one in the demo version! God, I want this game