I have identified an error made when processing the VAT element in a document from a certain supplier in every month between February 2009 and December 2012. The current VAT quarter ends on 30th June, and I am going to post an adjustment in the VAT accounts this quarter. The aggregate value of the error is below the threshold for declaration to HMRC. However, I am unsure as to what the HMRC actually means and need to know can/should the errors in the VAT periods prior to the period ended 30th June 2010 be adjusted in this quarter's return?

Anything previous to the four year limit is time barred for this kind of error, so you only have to correct errors going back four years. See Public Notice 700 / 45 section 4.6. You can still inform HMRC that you have just discovered that the error actually goes back to February 2009, but you are only making an adjustment going back four years due to the four year rule quoted above. Explain that the error was a real mistake and not deliberate.