Hey, I posted my timeline in another post, but what I wrote along with it is relevant to this post:
(....) I didn't include a letter from an accountant
because I do my own accounting. I also didn't include any self assessment forms
because I have yet to make one (mine's do next year). I did, however, include the letters HMRC sent me with my UTRn and my NIN, and also a couple of print outs of emails I got when I registered for online self assessment.
Other then that: personal bank account statements for 3 months, invoices that match the money going into the account, and a letter from my biggest client where I get most of my income from, stating that I work with them on a freelance basis and on an ongoing work relationship. I also included a printout of my portfolio (I'm a designer) just to tick the box with the whole "advertisement material" thing stating that I use it to get work. I sent a cover letter along with the application explaining everything regarding my business and why I wasn't including things like HMRC Self Assessments or NIN Contributions (I still haven't gotten a bill for NIN Contribution, I believe It's in april) and summarising what supporting documents I did include and why.
Hope this helps, and good look with your application!