Proof of living

[Linaco]

Hi all,
Could someone please clarify what is acceptable proof of living in the UK? I read the guide and the booklet and this is still not very clear to me.
I can see in the Documents section (I am applying online) the following statement: "Bank statements or household bills are not suitable proof you've been living in the UK".
Are only payslips and Self Assessment forms acceptable proof of living in the UK?

I've been permanently employed for 9.5 out of last 10 years and I have P60/P45/payslips to prove that. But since July 2017 I opened my limited company and worked as a contractor.
My questions are:
1. My limited company is not a registered employer, so I do not have a tax reference number for it. Shall I leave this space blank?
2. As a company director what proof of living can I provide? I have company registration certificates from Companies House and Corporation Tax/VAT Registration certificate from HMRC. I also have weekly payslips from the recruitment agency for the time worked as a contractor. As my tax return is only due in April, I do not wish to wait till then, as I qualify in February. Can I send these?
3. Do I need to submit personal bank statements to show receipt of salary when I was employed and business bank statements to show receipt of paid invoices in addition to P60, etc.
4. What is the period I need to show proof of living - 5 years?

thank you so much for your advice!