Self employed extension query very confused pls help

hi i am in tier 1 general migrant visa since 14 may 2009... i have to extend my visa by next year may 2012. Now my query is i am a sales person doing door to door sales for a company and i am self employed on commission based. i will be earning about 27000 pounds in commission as thats what i showed as my earning when i was employed and i applied my tier 1 visa. I want to know if its all commission what would my earning be considered is it 27000 or whatever its after tax that means 20% tax on 27000 and less the tax net earning. I am fully confused and i need help badly as i am loosing time day by day and soon my extension will come close. if i need to show 27000 for extension how much should i earn please let me know. is it net 27000 i have to earn ? or gross

i have already registered self employed with hmrc since 15th march 2011

Also do i need to open a company ... is it mandatory ? If yes how do i do that how do i register can i get a link please. If i am the only person in my company and just work from home i mean my office address will be home is that fine.

currently i get paid thru BACS in my personal account and my sales invoices are in my personal name.

I know its lots of questions but i really need help from seniors here.. pls

thanks in advance

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