How do I set up a disallowable expense in Simple Start? I want to record the payments I am making to HMRC for tax/NI contributions but they keep showing up on P&L.

I'll explain why the amount of the disallowed expense appears in the QuickBooks Profit and Loss (P&L) report, @juliemcguigan. This way, I can guide you accordingly. 

I'm happy to go over each step in entering the disallowed expenses in detail and share a workaround for keeping the disallowed amount from showing up in the P&L report.

The P&L statement summarises your company's total income and expenses at any given time. This report will include all of your income and expense transactions, including the disallowed payments. That said, it's customary to show the amount in this report.

To record the amount, follow the steps below: 

1. Go to the Plus icon and choose Expenses. 
2. Choose a Payee, Payment account, Payment date, and Payment method. 
3. Pick the appropriate category and enter the amount. 
4. Click Save and close. 

I advise customising the report such that the disallowed account is not displayed in P&L. To complete this, follow the instructions below:

1. Go to the Reports tab and open P&L. 
2. Click the Customise button and go to the Filter section. 
3. Change the Distribution Account or uncheck the Disallowable expenses box. 
4. Hit Run report. 

To save your current customisation settings, I'd recommend memorising the report. For more information, read this article: Memorise reports in QuickBooks Online.

Check out this post for more information about allowable and disallowable expenses in QuickBooks: 

Allowable and disallowable expenses are explained. 

If you want to do your own self-assessment taxes in Self-Employed in the future, I'll include this guide: Self Assessment for Income Tax checklist for QuickBooks Self-Employed. 

Please let me know if there is anything else I can do about your account or follow-up questions about disallowed expenses. Always ready to help you again.