There is a guide to complaining on this site: https://www.leaseholdersforum.org.uk/guides/complaining-effectively/ which suggests using the complaint form accessed via this page: http://www.camden.gov.uk/complaint (click the Make a Complaint button).
I don’t have experience of using the tribunal so can’t comment on that, but I do have invoices currently in dispute directly with the council and they have put them on hold in the meantime, which means that I am not chased for monies unpaid. Those invoices are for major works which overran and were poorly carried out. I began a dispute in late 2015 and it is ongoing. We have got nowhere for about 18 months and Camden have now transferred it to the complaints department.
Camden’s usual process when invoices are unpaid is to send standard letters to threaten legal action, although I don’t know how it would escalate from there. Perhaps others on this forum could comment?