Overall your installation seems ok it's easy to criticize and we all have our working practices , I personally don't think the use of plastic pipe is a cause of air ingress it looks like speed fit pipe and fittings that have been used which are reliable and have been used widely for years , any pipework around the cylinder hot or cold should be insulated as should any pipework in cold areas and voids , your heating pump will be inside your boiler casing the property seems like a large family home and a system boiler with a internal pump wouldn't of been my choice as these boilers struggle to move the volume of water in the system and absorb the expansion there are ways to improve this and I can see why maybe it was fitted because there is not much space , has the system always been problematic ? is the work completed ? we could advise ways to improve your system but it's really the installers responsibility to make sure its commissioned and working correctly you plan to extend your home further and this without doubt will cause more problems unless improvements are made . Regards kop