I have foolishly deleted my bank account in QB, and lost all of the previous tax year data.

This must be a challenging experience for you, @Barney.

Currently, the option to recover your lost data once you delete your bank account and lost your data in QuickBooks Self-Employed (QBSE) is unavailable. 

I understand the importance of recovering your data in QBSE. For now, we have to link your bank account again and manually upload or import your transactions. 

To connect your bank or credit card account, here's how:

1. Select the Profile icon and then select Connect Bank.
2. In the search box, enter the name or URL of your bank. Then select Continue.
3. If you've already connected an account before, select Connect Another.
4. Enter the sign-in info you use for your bank's website.
5. Once done, select Connect.

For reference, check this article: Connect bank and credit card accounts to QBSE.

To manually import your transactions into QBSE. Follow the steps below:

1. Open QuickBooks Self-Employed in a web browser.
2. Select Settings, and then click Imports.
3. Select Import older transactions for the account you want to add transactions to.
4. Select Browse and find the file you downloaded from your bank. It's usually in your Downloads folder.
5. Select the file and select Open to start the import.
6. Review the column headers and data. Make sure the Date, Description, and Amount columns on the CSV match the columns and order in QuickBooks. The other column headers can be in any order.
7. Select Continue to complete the import.

For more information, refer to this article: Manually import transactions into QBSE.

I'll also add this article to help you review and download your transactions: Export transactions and get reports.

Feel free to click the Reply button below if you have any other questions about recovering your lost data. I'm always here to help. Have a great day.