Katherine's flat is well located : Brixton underground station (Victoria line) and plenty of buses to go from the station to her flat.
She's also very available by email or phone, gave us all necessary details : this an important point for us, we've been a couple of times in Spain, all owners were British citizens : each time, we've been very well welcomed ;-)
This was the first time for us in London with our kids, but pay attention if you plan a journey during X-Mas : London is a day off, be prepared to walk ;-)