When to apply VAT adjustments?

I set the VAT start date as 1 March 2013 and have obviously submitted the VAT return after that, but I have an adjustment to enter which goes back to a date prior to this - if I enter it now, will it get included in the next VAT return or do I need to enter it with a date past 1 March 2013?

Any adjustment you enter will be applied to the VAT return you’re filing. HMRC don’t see these adjustment they only see the NET figures on the return. As long as you can substantiate these adjustments should they later investigate.