Home DIY

I usually use Rentokill, they usually charge a flat fee and get the job done. Only because there are so many cowboys in the drain clearing business.

It's worth carrying out a bit of investigation yourself if you can. Maybe even poke around a bit. What are the symptoms?

So not DIY... but can anyone recommend a drain unblocking company that serves North London?

Good experience, albeit years ago (at least 15) with Burch Services - https://www.burchdrainage.com/

Obviously try all the DIY suggestions first!