From what I can tell online, the 39CPL seems to be highly customizable, so troubleshooting can get a bit complicated if we don’t know what range of options have been installed with it. Have you tried getting in touch with the manufacturer? Maybe they can send someone over to help you.
If you’re pressed for time, a local air conditioning repairman might be able to figure out what the problem is. There appears to be a lot of them in Staffordshire, but I see a company that deals in air conditioning in Burton-on-Trent
that may be able to help you with your fresh air system. Go online and search for BRAC Engineer Limited. Call them and ask if they can come take a look at your customized 39CPL, and if they can repair it for you. Hope this helps!