The BIA website definition for Previous Earnings is as follows:
This is a category for which you may score points under the points-based system for coming to the United Kingdom to work, train or study. It means the amount of money you have earned in 12 consecutive months during the 15 months immediately before you make your application.
Furthermore for claiming points under United Kingdom experience, the website states that:
You are awarded five points if you are applying for permission to enter the United Kingdom (known as 'leave to enter'), and £16,000 or more of your previous earnings that you are claiming points for were earned in the United Kingdom.
I was in the UK till August '07. I plan to make my application in April '08. For claiming the 5 points for UK experience, I need to submit atleast 6 months of my UK payslips. My question is should I submit payslips from Jan '07 to Dec '07? Or is it required to submit payslips for all 15 months from Jan '07 upto March '08?