Good work, Thom!
Your vocals remind me of the "Crash Test Dummies". Very distinct.
As an exercise, try carving off the low frequencies from James' vocal (say, a 15 dB cut on everything 250 Hz and below).
If ever you have the opportunity to work with some pitch correction software, you might want to apply it to those portions of James' performance where he's singing with you or Martin - just those areas where there is pitch-related dissonance.
This mixing/mastering thing is very "learnable", and you have an affinity for it.
It's only going to get better
Oren.