We emailed Building Control, who said, "Building Control do not offer a guarantee, our completion certificate is issued on the premise that as far as we can reasonably ascertain, we believe the property to be compliant with the building regulations." and "We will have checked the air bricks at an early stage in the build, when the ground floor was being constructed. Landscaping etc sometimes occurs after our final inspection, and they may have got covered over during this stage. It is the builders responsibility to demonstrate compliance with the Regulations and to ensure the building is handed over to the owner in a condition that is fir for purpose." This is simply not true, as patio was already done when they signed off the certificate.
We also contacted the warranty provider, who suggested we "would need to raise these issues with the developer in the first however, If you feel dissatisfied with this you do have other options such as taking legal action or contacting trading standards." and "submit a claim after the 2-year period for defects".
It seems that both Building Control and Warranty do not feel responsible...