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The easiest option would be to split the file in two and upload the pre-VAT transactions first - if you have already enabled VAT (by entering details within the 'Taxes' tab) you can create a bank rule to automatically assign all purchase transactions to 0% by selecting the 'Apply this to transactions that are' 'Money out' 'and include the following' 'All' 'Amount' 'Is greater than' '0.01' and select the appropriate tax code on the drop-down below.
If you haven't yet setup VAT, leave this turned off and turn on after uploading the pre-VAT transactions.
After uploading these, delete the first rule (Banking > Rules > Select the drop-down in the action column > Delete) and then create a new rule with the same details above, instead selecting the 20% S tax code on the drop-down. You can then import the post-VAT registration transactions after this. 🙂
Ah! Thanks for getting back to me usertom5, if the transactions are in the for-review tab you can batch tick the pre-VAT transactions by ticking the check-box to the left of each and then select Update > Select the applicable VAT field on the drop-down of the pop-up box. 🙂
^Just to make things easier, you can also select the drop-down beside 'All dates' in the top-left of the For-Review screen to enter a custom date period, and then select the top check-box beside the date column-top batch-select all. 😉