If you have deregistered for VAT as you think that in the next 12 months your turnover will be under £77000, but say if in 10 months time your turnover is better than expected and have to reregister. The question is would I then have to backdate any VAT due back to when I de-registered??
If you are required to re-register, you can ignore supplies made under your previous registration.
The legal ref is VAT Act 1994, Sch 1, para 1(4). Published guidance is well-hidden at V1-28, para 13.17. This is accessed from this link: customs.hmrc.gov.uk/channelsPortalWebApp/channelsPortalWebApp.portal
and, do retain a copy of this reply in your VAT file!