Mould in my flat and landlord says it's a 'lifestyle issue'

Black mould in the bedroom and bathroom of my flat in Canary Wharf area (E14). It keeps coming back no matter how much I clean it. I've got a dehumidifier running constantly and always open windows when showering.

Landlord's response? "It's a lifestyle issue — you need to ventilate more." The flat has single-glazed windows from the 1990s and zero extractor fans. The bathroom doesn't even have an opening window.

I'm paying £1,650/month for this flat and it's making me ill. What are my rights here?