This is the second time I've stayed at Siobhan's place and both times it has been great. Location is really handy for Brixton Station but the flat is self is just brill. Real homely feel too it, some lovely touches and artwork, super comfy beds and a great shower.
You need to locate the key to get access - it's on a lockbox attached to a nearby bike rack. The instructions you'll get to find it are really good though. It was in a different place the second time but still straightforward. Just look carefully if there are bikes though as it may be obscured slightly.