Thank-you for your question. Because the process by which you are paid is not straightforward, I suggest that it would be sensible to explain this process to the UKBA. You can do this by simply submitting a covering letter with your application.
Regarding the documents that you need to submit as evidence of your previous earnings, are you familiar with the Tier 1 Policy Guidance? This is a document written by the UKBA for its caseworkers that sets out all the detailed requirements for a successful Tier 1 application. You can download a copy here:
I suggest that you have a look at paragraphs 137 - 146 generally. However, most importantly, you should note in particular the final section at paragraph 146 which specifically addresses the documentary evidence of previous earnings that should be provided by contractors. This states:
If the applicant is a contractor who does not operate either through his/her own company or as an employee, he/she may provide:
an accountant's letter confirming a breakdown of his/her gross and net earnings for the period claimed; and
personal bank statements clearly highlighting all credit payments made to his/her account from employment undertaken during the earnings period claimed.
Please note that both pieces of evidence must be submitted with your application.
I hope this answers your question. Please kindly click the green ACCEPT button so that I may be compensated for my time. Thank-you and good luck with your application.