Step one: Raise your concerns with the trader directly
Step Two: If they cannot address your complaint to your satisfaction you can escalate the matter to the Glass & Glazing Federation or FENSA as appropriate.
Step Three: If intervention from GGF or FENSA does not resolve the issue you may apply to TGAS.
Centre for Effective Dispute Resolution
70 Fleet Street
Telephone: 020 7520 3800
Fax: 0845 1308 117
Textphone: 020 7520 3767