How do I record VAT calculated to the end of April when I set up QB to run from the start of May?

I'm delighted to see you here, RobW1,

We can increase your VAT Liability in QuickBooks Online covering the balance for April. To do that:

1. Go to the Taxes menu and select Prepare return for the period you want to submit.
2. In the VAT line, select Adjust.
3. Enter the Adjustment Date and VAT rate.
4. Pick and Adjustment Account from the drop-down menu▼.
5. Add the amount to adjust press Save.

If you aren't sure on the accounts to use, we recommend seeking help from an accounting professional for additional guidance. Also, for restrictions and other reminders before making any adjustments, please see this article: Adjust your VAT liability.

Alternatively, for more VAT and other QuickBooks help, check out our Community guide here.

Let me know if you need anything further. I'll be on the watch to provide additional help. Have a nice day!