I cannot see how QB calculates corporation tax and there is no information in the help section - am in the uk - any ideas?

Hello there, @beckypearsoncons.

Currently, calculating corporate tax is not available in QuickBooks Online. For recording purposes, you can create an expense or bill to record your corporation tax payments manually. 

To record your corporation tax payments:

1. Click the Plus Icon (+).
2. Choose Expense or Bill.
3. Select the name of Agency where you paid the taxes.
4. Enter the tax payment details.
5. Click Save and Close.

For more detailed instructions, you may check these articles:

• How to Enter and Pay Bills?
• How to Enter, Edit, or Delete Expenses?

Also, you man check a supported third-party app to track and calculate your 

corporation tax.

 Here's how:

1. Go to Apps.
2. Enter Corporation Tax in the Search field.
3. Select to open the app's overview.
4. Click Get the app now.

For your reference, you can read this article to learn more about integrating an app: QuickBooks App Store.

Lastly, I'd consult contacting your Accountant or a Tax Professional so you'll be guided accurately in calculating your corporation taxes.

Keep me posted if you have other questions. I'm always here to help.