If your debtor is a business then the winding up of its affairs will be being dealt with by an administrator.
The administrator will work out what assets the company has and how much can be paid out to its debtors, if anything. To find out the name of the administrator you can contact Companies House who will be able to assist.
You should then write to the administrator registering the details of your debt with them so that they can include this when they come to divide up the assets. If the debtor is an individual then their bankruptcy will be being dealt with by the Official Receiver or an Insolvency Practitioner and again you should contact them to register your debt to make sure it is included should there be any assets to be distributed.