Tax Return Query United Kingdom?

[buster]

I received a letter from HM revenue and Customs (Local Compliance) yesterday because I failed to declare on my tax return (2006/07) that I worked for, apparently, 2 other companies, although I was with the same management team
Well, to be honest I assumed the figure on the P60 took everything into consideration. Now I have been overpaid £458 by HM Revenue & Customs. The figure I entered on previous year's tax return (2005/06) was the bit where it states 'In this employment' The figure for tax return (2006/07) was different, but still entered the info from that area of the P60. I've always used this figure and it's the one the Inland Revenue tell you to enter on the return I was I was advised by my previous that I should have entered the figure from the 'Total for the year' section of the P60 on to the tax return. I did argue that I have never had to do that before. Apparently, I should have been issued a P45 from both companies plus dates when all the changes took place. Well I didn't