Readers' forum: Antique claims

I have just completed a final VAT return for an antique dealer who is deregistering. Until alerted by our tax manager I did not realise that I did not need to declare output tax on stock owned by the business when it deregistered because input tax would not have been claimed on this stock when it was originally bought by the business (under the VAT margin scheme). Fortunately the error was discovered before I submitted the return for that period. However when I reflected on the situation I realised that I made the same mistake on the final VAT return of another antique dealer I deregistered in April 2016 the VAT overpaid was £3 600 and this return was submitted to HMRC. The business was a husband and wife partnership and they now live in Spain. I would like to correct the error if possible...