When I complete an invoice it goes straight to paid showing the balance as zero!This only started yesterday and haven't had a problem before

Glad to have you here in the Community, @redcliffebayelec.

I'm here to provide information on why invoices automatically get paid without receiving the payment.

One of the reasons why invoices are paid automatically is that the Automatically apply credits option was turned on. Let's check this option from the Account and Settings and turn it off. This way, QuickBooks won't apply credits to the newly created invoice. Here's how:

1. Sign in to your QuickBooks Online account.
2. Click the Gear icon, then select Account and settings.
3. Select Advanced from the left menu, then go to the Automation section.
4. Toggle the Automatically apply credits option to turn it off.
5. Click Save, then Done. 

Once done, you can remove the applied payment on the created invoice by deleting it. To do so, you can open the invoice, then click the More button and select Delete.

If you're ready to receive the invoice payment, you can check out this article for more guidance: Record an invoice payment.

This should resolve your issue, @redcliffebayelec. Come back to this post if you get the same result. I'll be around to provide further assistance.