In 2015 the chance of getting away with working on the side are incredibly unlikely. They get a slap on the wrist, you get a HUGE FINE, If you are serious about been in business, act like it and do things right for your own good, you WILL get caught.
You can hire them on a self employed basis, which in Domestic cleaning is very common, tell them they have 90 days to give you a UTR number, this way you are covered if you end up been inspected., reported, grassed on etc etc.
If you need a template for hiring them, just let me know I think I have one somewhere.