Garnishee Order
You can use a garnishee order to force a third party who either holds money belonging to the debtor or owes the debtor money. The garnishee order only works once at the moment it is served. If there is no money in a bank account at the time it is served then the order fails.
To apply for a garnishee order you need to fill in a copy of Form N349 and send it to the court with the appropriate fee.
See also: EX325 - Third party debt orders and charging orders - how do I apply for an order?