Hi Paul
Sorry to hear about your accident. Do you have a solicitor? If not you may want to think about getting one. They work for free on a no win no fee basis and will deal with all of this for you. Yes indeed the insurance company can go to the HMRC for your records with your consent. In order to prove loss of earnings as a result of an accident when you are self employed however you have to not just show the income previously. Indeed the work you could not do after the accident is far more important.
Even if they have your past earning details being self employed they will ask you to prove you lost work because of the accident. The best way to do this is letters from your clients or potential clients at the time saying had you have been fit for work you would have been able to work for them at what ever rate they would normally pay you.
So get letters from your clients saying they would have given you work had you have been able to work. This is much more valuable than details of your previous earnings.
I hope this helps
