that's exactly what this forum was set up to avoid, but it happens occasionally. Think your question has been pretty well answered, would be interested to know which way you noe lean in regard to codes?
Get the customr to make a formal complaint to UKAS .... clearly the offending contactors have not been assessed to the correct standard.
Remember, even if the NICEIC DO uphold a complaint, they will make NO effort to ascertain the extent of said offenders non-compliant/dangerous work, so previous customers will receive NO warning of any potentially unsafe work carried out in their property.