You should try and contact property managers or factors. They're the type who will be looking for regular cleans at managed properties/estates.
You get firms who are property managers AND letting agents, but letting agents are the folk who know a flat is going to be vacated at least two months before, cos they've got to get the paperwork organised in advance, and then phone you in the morning saying the new folk are moving in that afternoon, you've got to drop everything else you're doing to keep them happy, and if you don't they never pass you any work again.
Oh, and they always pay as soon as your invoice goes in - NOT!
Do I sound biased?