I wrote letters to
escalations@betfair.com
But their answer was not much different from that of a regular support service.
How can I prove
that the funds in the account come from the source shown on the tax returns at a guess.
They do not write anything about it. They simply write that the documents submitted are not enough.
The last letter from Betfair support (the Russian-speaking support service answered. Translate by Google):
Hello, Andrey! Thank you for your appeal. As part of the audit, we ask you to provide additional information and supporting documentation regarding the financing of your Betfair account, namely a full explanation of how you finance your account (for example, salary, savings, property, or other income), as well as documentation that confirms this (for example, a document on salary / property, etc.). The documentation you submit should reflect the relationship between your money and receipts to your Betfair account, that is, there should be a clear line that when you replenished your account with Betfair during the periods of 2015-2018, you had money and where did you get it from. Unfortunately, the documents presented earlier are not enough because these documents do not reflect the availability of sufficient funds to fund your Betfair account.