My personal experience says give Camden Repairs a call and they’ll tell you whether they think this is a job for them or not; best dealt with on the phone however rather than online as it’s a complex query – you get a response immediately and also can organise for the plumber to come round at a time that suits you (if Camden are willing to investigate further). If they say it’s a job for them problem solved and stress removed.
If they say that it’s not, then you can investigate the options. As it’s the main stopcock I’m guessing that you could have an argument either way. A *draft* edition of the current leaseholders repairs guide that is being worked on says: “Pipe work that relates solely to the property such as a leaking waste pipe, trap etc.” is the leaseholder’s responsibility; I would say that as the stopcock relates to/separates from the communal infrastructure then it’s a job for Camden. Only one way to find out – and Camden will have had this query before so it won’t be anything new for them.