I phoned the locksmith at about 11:00, and was told that he'd be here 45 minutes later; he eventually turned up at 13:30. Fortunately I'd already got into the flat by then, so I was just curious about what had happened to him - it turned out that he'd got stuck on a roof, when someone at his previous job had said "Back in a minute", then disappeared off for half an hour, and the communal door had slammed shut... Anyway, I got the lock replaced, so now I just need to dig out my spare set of keys.
As for the plumbing, I had a visit from Thames Water mid-afternoon, and the guy said that the mains outside wasn't jammed - he confirmed that it was the right one for my flat, by turning it on and off. So, at least I've learned something. I then got the original plumber to come back after that, and he had a look at the stopcock in my flat, in case there was a broken washer. Unfortunately, it looks as if the problem is more serious than that - there are rusted pipes in the basement of the shop underneath me, so I need to get a new pipe put in from the water main. This is going to be slow, expensive, and will involve knocking holes in my walls/floor. Annoying timing too, since I'd finally got around to doing my decorating.
The cost is a slightly complicated issue. The pipes go through the shop, but the shop actually have their own separate water main, i.e. it's not their problem that my water's on the blink. They're being very friendly about allowing me access, but realistically they're not going to pay for repairs. If anything, it will be the other way around - someone will need to pay them to go digging through their walls. At the moment, this water main supplies me and the flat upstairs, so I'll talk to the owner of that one (non-resident landlord) - this may be a good time for him to sort out a separate water main for them, or we can look at splitting the cost of the repair. The more promising alternative is that since we're both leaseholders, the freeholder may actually cover the cost of this. I need to dig out my contract, and check on that, then I can talk to the relevant solicitor tomorrow morning.
Meanwhile, the pressure has had another drop - it now takes over a minute to fill a pint glass with the kitchen tap on full (I don't know how long it takes altogether because I gave up at that point). So, I'm now off to the gym in search of water (swimming pool/shower), and I'll do the yuppie thing by buying bottled water on my way back afterwards.