HSMP - Tier 1: Self Employed / Contractor query

[Sphinxed]

Hello experts/members/sages,

Please, I need urgent clarification on this challenge -

I intend to apply for HSMP -Tier 1 extension soon using PEO.

I have a main daily job BUT my earnings do NOT meet my target, hence the questions below -

Can I register as IT self-employed / contractor?

Can "proof of earnings" from being self-employed /contractor be -

(1) An accountant letter stating that I am registered as a Self employed with my income generated (before and after Tax) ?

(2) My invoices generated to my client (s) ?

(3) My Personal Bank Statement (same as the one used for my other job) reflecting client's payments before Tax ?

Also, is there any extra information I might need to provide ???

Your usual useful advice(s) / response(s) would be most appreciated.

Thank you