If the Business/Trades Person no longer holds certification then NAPIT have no authority to address your complaint with them. 

The Business/Trades Person is still subject to the contract entered into, which should have committed them to assuring compliance for a period of six years. 

You should still complain to them and remind them of their obligation, they should resolve the complaint. If they fail to resolve your concerns, we recommend that you contact your local citizens advice, trading standards  or seek legal advice to query what other routes are available to you.