If it was/is a limited company you may be able to get the home addresses of the directors from Company House in Cardiff, this used to be free but they now charge a couple of quid. When you have their home address write to them giving 7 days to pay before you issue court proceedings. It might be enough to frighten them into paying up without the drawn out rigmarol of a court battle.
Do it NOW, if they are going into receivership you might never get paid.
Bad luck by the way, it happens to all of us at some point.