FENSA's remit is limited to issues concerning Building Regulations compliance, so does not extend to issues related to contractual disputes with FENSA Approved Installers. (See: Criteria for complaints that may be referred to FENSA)
As with any issue you may have with an installer, we encourage you to first attempt to resolve the matter directly with the installation company.
If you require further assistance in seeking a resolution, we recommend contacting Trading Standards.
Alternatively, if your installer has opted in to an Alternative Dispute Resolution service, such as TGAS then you should contact this service directly.