New Account wizard, Financial page; selecting cell from Instalments button does not work if overriding payment entered

On the New Account wizard, Financial page, selecting a cell from the Instalments drilldown does not work if an overriding payment has been entered.

To recreate:

1. Run the New Account wizard to the Financial page and calculate.
2. Enter an amount in the payment field to override the regular payment.
3. Click the "Instalments" button.
4. Click on a cell to update the Financial figures.

The Overriding payment value is not cleared and therefore does not work as expected.

The built-in SummaryPage_AccountInstalments_InstalmentSummary script has been updated to clear the Overriding Payment when an instalment has been selected.