I agree with Melanie that that whilst the wording is clear, there is a discretion on the supervisors part. 50% of "additional income" could be net or gross - like Melanie if I were your Supervisor it would be net.
If you wanted to make a complaint it would be against your Supervisor personally. All IPs at the firm you mention are licensed by the IPA. The e-mail address of your Supervisor is available from here:
http://www.insolvency-service.co.uk/newipsearch.htm
I would send your Supervisor an e-mail setting out your concerns and the fact that his staff have failed to reply to your requests for a meeting. Also indicate that unless your queries are addressed you will have to consider complaining to his licensing body. That will I can assure you, have an effect!
Ian
Oops! Whilst I was typing this you posted your most recent message!