I'm guessing that you bought it at Perry's (because I don't remember seeing any other bike shops in Woolwich) and I was wondering whether they'd be open today. I remember wandering about one Sunday somewhere in Thamesmead, coming across a couple of young kids trying to sort out the brakes on a bike, and patching them up on the roadside so that the bike was safe to ride for a while. I asked them where their nearest bike shop was, they said Perry's, and later, as it was on my way home I stopped by, and found the shop shut - plus, there was a note in the window saying that repairs would take 2 weeks! (It was midsummer, though.)
From my limited experience, I would say that it's rare for a hangar to shear without undue stress. They're designed to shear but that's to protect more sensitive/expensive parts in the event of an off. If the frame and dérailleur are in good condition and the bike was running well up to that point, then I'd say it's simply a matter of replacing that part without it being a problem that bears on the bike. The problem, though, is that you've paid for a bike that you can't ride or take back in during the week. But if you were happy with your bike then the problem is solved by a simple replacement and it's no reflection on the important bits of the bike. The thing that was designed to be weak was too weak...
Mind you, if I were a bike, I'd make you really struggle to get me to Bluewater