Chris, I feel your resentment.
However, is it possible he was listed on the copyright papers as arranger, not composer?
This would enable the publishing company/record company to legally leave out his name on any credits, as arrangers are not entitled to writing credits (which is unfair I think).
Anyway, 7,5% for a bass line is pretty good. Well done!
Who did you make the deal with at MoS? I have something signed (The Whistle Song) with them so I know quite a few of the people there.