My opening balance is +£120 and ending balance shows -£120. The debits and credits are correct as per the bank statement, How do I correct this.

Hi there, RHS. 

The ending balance is the amount you can see on your bank statement. If you're in the process of reconciling an account and mistakenly entered the ending balance amount, you can edit it in the Reconciliation window.

Here's how:

1. Click the Gear icon in the upper-right corner.
2. Below Tools, select Reconcile.
3. Choose an account.
4. Hit Resume reconciling.
5. Press the Edit info option in the upper-right of your screen.
6. Edit the ending balance.
7. Click Save.

If you need to change the ending balance from the previous reconciliation, you need to manually unreconcile the transactions. Then, reconcile that period again entering the correct statement balance. Before doing so, I suggest consulting your accountant, so they can help you through the process. 

Let me show you how: 

1. Go to the Accounting menu and click Chart of Accounts.
2. Click View register from your bank's register.
3. Select the Filter icon above the Date column to only see the previous transactions that were reconciled. 
4. Repeatedly click R (reconciled) until it's blank (unreconciled). 

Once done, you can start reconciling the account again. For more insights on how to fix the ending balance issues and errors when reconciling, please refer to the following article: Fix issues when you're reconciling accounts in QuickBooks Online.

Feel free to post again if you have questions. I'll be happy to help you some more.