It depends greatly upon how your interaction with the service has come about. If you are the parent or person connected to a child who has become looked-after by the Local Authority, then your recourse is through the family court. If you are a foster carer or other party, then really speaking your first port of call should be the Council’s internal complaints procedure, followed by representation to the Public Service Ombudsman for Wales.
In the very first instance I would suggest 1) putting a call in to the CAB for advice and 2) getting in touch with Lesley Griffiths, who is well versed in complaints against the Wrexham fostering service.