I had seen that if I selected the "Straight corners" option, then it actually would produce that "Fillet-but-leave-the-original-surface-alone" result, which would allow me to do the lower level surface work if I really needed just that edge filleted... :o

Thanks to Michael, Danny and BurrMan for their help with which I was able to resolve the issues. I have realised that some edges just can't be filleted but I managed to get a useful result nonetheless. Cheers, Dan