I'm a sole trader and just paid my self assessment tax. Where in QBO is this recorded?

Help is finally here, RobB81.

You can create a Journal Entry (JE) to record the payment without affecting your profit and loss. Before doing so, I suggest consulting an accounting professional to provide more expert advice on how to handle this situation. They may suggest setting up a chart of account to show the amount of tax owed to HMRC(likely as a liability) Then you can create a journal to reduce the liability and the bank account. That would then show on the balance sheet but not the p and l

Here's how to create JE:

1. Select + New.
2. Select Journal entry.
3. On the first line, select an account from the Account field. Depending on if you need to debit or credit the account, enter the amount in the correct column.
4. On the next line, select the other account you're moving money to or from. Depending on if you entered a debit or credit on the first line, enter the same amount in the opposite column.
5. Check the amounts - you should have the same amount in the Credit column on one line and the Debit column on the other. This means the accounts are in balance.
6. Enter information in the memo section so you know why you made the journal entry.
7. Select Save and new or Save and close.

For additional information on this, I recommend checking the following article about creating Journal Entry: Create a journal entry in QuickBooks Online 

In case you want to review the entries made, you can pull up or print a report for Journal Entries

To take a closer look at your business finances, you can customise your financial reports. Check this article for details: Customise your company and financial reports.

If there's anything else that I can help you with, feel comfortable inserting a comment below. Have a great rest of the day.