I came across the same problem when I first made enquiries about starting up an 'agency' with the cleaners being self-employed. I spoke to a Solicitor and was told that if the cleaners I sub-contracted worked the same hours on the same day for the same customer each week, fortnight, month, whatever, then were classed as employees, not subcontracters. In which case if I sent them out as sub-contractors this way and the good old IR caught up with ME, I would be liable for NI and Tax contributions. So I decided it was not worth the risk.
And have you thought about liability insurances. What would you do if someone injured themselves because of something your cleaner did, or didn't do?? Or broke something of value. You can get your own public liability insurance, but what about the cleaner? I have public and employers liability insurance and I sleep better at night for it!!
Incidentally the IR are fully aware that there are cleaning 'agencies' employing 'sub-contactors' to clean for them, and when I visited my Solicitor (which was about 6 months ago now) he did mention that the IR were planning on cracking down on this way of working., and I also heard that there were going to be certain rules and regulation set to stop people like us working that way ........just a point of notice, dunno if of when that is likely to happen tho'