Though not in London we do use cleaners for 3 hours a week, paying £35. It helps keep on top of things as we both work have two dogs, two teenagers (though one has gone to Uni).
They are great, we let them have a key, they spoil the dogs, and do a good job - have had them for about five years now.
The best bet is word of mouth, do you know other folks that have reliable cleaners they could recommend.