Can anyone advise me how to close off end of year accounts for the tax year on QuickBooks, also can this be done in retrospect once all relevant invoices are paid?

Closing end-of-year accounts is quick and easy. I'll guide you through how.

The year-end closing process identifies any balances or deficits on the company's books, which are then carried over into the new fiscal year. This ensures everything stays the way that you want and prevents any accidental changes that may affect your financial reports. You can review your accounts before you close your books. To review your accounts, follow the steps below.

1. Sign in to QuickBooks Online as a primary or company admin.
2. Review your accounts and make sure everything looks good.
3. Enter any outstanding invoices, expenses, and payments.
4. Reconcile your accounts up to your closing date.
5. Review your stock quantities.
6. Finally, you can use this guide if you may need to complete specific tasks in QuickBooks Online: QuickBooks Online Year-End Guide And Checklist.

After that, you can now close your books. Here's how:

1. Go to the Gear ⚙ icon and then select Accounts and settings.
2. Select the Advanced tab.
3. Select Edit ✎ in the Accounting section.
4. Turn on the Close the books switch.
5. Enter a closing date. Give yourself a comfortable deadline. You shouldn't need to edit any transactions before this date. Your new bookkeeping work will start after this point.
6. Select the Allow changes after viewing a warning and entering a password option from the dropdown menu if you want to require a password before editing your closed books.
7. Click Save and then Done.

It is also worth consulting with an accountant for your year end.

Let me know if you have any concerns about year-end. I'll be happy to help.