just to be straight, a certificate of compliance is not an insurance or guarantee against faults, if it is shown the faults are not caused by something required by the building regulations.
A cert of compliance is underwritten by a building professionals PI (professional indemnity) insurance.
The example you give above is not specifically referred to in the building regs. It may come under the 'workmanship' reg, but trying to proove that would be a nightmare. As the house is 6 years old it could be more appropriate for this fault to be considered a 'maintanence' issue. Every house needs constant maintanence to provide a sound habitat for its inhabitants.