Just for anyone else, we may have sorted this now. We created 4 Gen jnl lines with posting date with no transactions.
sales GL account | £120 | with £20 VAT (20%) | Balanced against a GL account (VATCONV)
Sales GL account | -100 | 0.00 VAT (Remove VAT Postings) | Balanced against a GL account (VATCONV)
Purchase GL account | -£72 | -£12 VAT (20%) | Balanced against a GL account (VATCONV)
Purchase GL Account | £60 | 0.00 VAT (Remove VAT Postings) | Balanced against a new GL account (VATCONV)
then run VAT statement for same day.. should see correct figures..