If they are in employment, don't own their own property and are unliely to have any significant assets (which if they live with their parents, sounds likely), I think the best way is to get an attachment of earnings order - which basically gives a mandate to their employer to deduct the money you are owed from their pay packet. You can compel the debtor to attend court for questioning in order to obtain their employer's details, as well as bank account details, details of any assets they have and any other information needed in orrder for you to decide which is the best way to get the money you are owed.
you can read about all of the options here:
http://www.hmcourts-service.gov.uk/infoabout/e nforcement/index.htm
I can personally recommend getting them to attend court for questioning, you can be present if you want and put questions directly to them. Great fun, watching them squirm!