I think you’re on the right track. You’re clearly not getting the service you’re entitled to, so I would start with an official complaint. I’ve been successful using that approach as they have a separate team that will investigate the complaint. I usually email them on firstname.lastname@example.org and complete the form available here… https://www.camden.gov.uk/complaints
Just be aware that it can take some time to get a response even from the complaints team, at the moment up to 20 working days. So if the issue is urgent you need to find an alternative route.
Have you also contacted leaseholder services? If not, I would contact them first as they are ultimately representing the landlord.
It might be possible to go and see them in person on Judd Street, I have never tried. Don’t know if you need to make an appointment.
Not sure about infringement. I doubt they’re in breach with the leasehold agreement. Suggest you seek legal advice if you want to go down that route.