Thanks everyone for replying. Just an update to let you know how things are going.

I have told the girl in Tottenham that I won't be renting the room and she was ok with it - she said she'd rather I lived somewhere where I personally felt safe. She admitted herself that at night she'll get a taxi home from the tube station rather than getting a 5min-bus from a stop in front of the tube, or walking 10mins along a fairly well-lit road. That in itself made me think I made the right decision turning the room down in the end. She thinks the taxi-arrangement is fine, but I don't want to live somewhere where I need to get a taxi in favour of a 5min-bus ride (and a 3min walk to the front door from the bus stop) or a 10min-walk to get home!!

The place in Southfields was perfect, but the girls in the end decided to rent the room to one of the other people looking round (although I was the first one to see it, and I made it very clear that I'd love to rent the room). I have been told that this is quite common, similar to a job interview - you don't offer a job to the first person you like and who says they'd take the job if offered it; you interview a few people and then make offers to the best candidate(s).

I was meant to see another place today that ticked all the boxes, but the girl emailed me this morning to say someone else had taken the room (ie seen it last week and taken a few days to think it over before saying she wanted the room).

I'm leaving room-hunting til exams are over now. I have a good handful of areas to look in now, having walked around various parts of London quite a lot. Thanks for all your suggestions of where to add/where to avoid, people!

Michelle, it's a real shame you're not renting to students anymore cos of your move - your granny annex sounds perfect!