I have paid an invoice twice in error. the supplier will refund one of the payments. how do i offset this with no invoice so as to enable me to reconcile bank?

I'll help you offset the amount, joe-lythgoe.

Let's make sure that the bill payments are recorded and the supplier's open balance shows negative. Please see this sample screenshot:

After you receive the supplier's refund, you can record the amount as a deposit. This will offset the open balance on the supplier's page.

Here's how:

1. Click the New (+) icon located above the Dashboard.
2. Under Other, select Bank Deposit.
3. From the Account drop-down menu, select the bank account where the payment was made and refunded.
4. Choose the Date.
5. Under Add funds to this deposit, choose the supplier's name from the Receive From column.
6. Select Creditors from the Account column. 
7. Enter the amount in the Amount field and click Save and new or Save and close.

I'm adding this sample screenshot for reference:

Once you have the transactions downloaded from your online banking, you can follow this article: Download, match, and categorise your bank transactions in QuickBooks Online.

Then, you can start reconciling the bank account.

I'm just a few clicks away if you have any questions or need further assistance. Thanks.