If you employ someone directly they are entitled to 5.6 weeks annual leave (pro-rata) which can include bank holidays.
You must also pay SSP (sick pay) after 3 consecutive days absence.
If an employee gets pregnant then there is a lot of maternity pay to pay for for a long time - this has not happened to us yet but I know it could.
You are also legally obliged to calculate NI and tax they owe and withhold this and send it off to HMRC (I have an accountant run our payroll as I cannot be bothered with all that, my business owner friends who do it for themselves say it's fiddly).
I honestly think cleaners working directly for a client should be self-employed and responsible for their own tax,/insurance/ business costs though, that is why cleaning is so expensive, they need to factor in all their costs then be left with an adequate wage at a rate which they don't mind cleaning on site and also doing all their personal admin and other business chores for.
I sometimes think when people regard cleaner rates (say £15 an hour) as a lot of money for a self employed cleaner they need to think about how much of a wage he/she will be able to draw when all costs of business have been taken into consideration. Margins are very tight in this industry all things considered, when operating legally and acting in customer's best interests at all times cleaning business owners actually struggle to be left with a good living, there is huge demand for services but it's fairly difficult to get the price you need to actually make a decent living. You have to have a passion for what your doing, it's not a 'get rich' industry. I love it though.
I think as long as you check that a solo cleaner is trading legitimately (ie HMRC document to prove registered as self employed and insurance documents) you should be fairly safe from dodgy types since people in business have a reputation to protect and will absolutely rely upon word of mouth to find clients in the beginning. I think it's ok to pay cash as long as they offer other options such as bank transfer or cheque so you know they don;t mind showing all their income to whoever. If there's only a cash option that smells like potential trouble.