I recently did tax returns going back 5 years for self employed person. He had called HMRC who sent him paper copies of the return for each year.
In his case he was actually owed a refund for each year (he was a CIS subcontractor) but we had to estimate some expenses. I made a note of exactly how everything was caluclated, and entered on the tax returns that some expenses had been estimated, giving the reaons why.