Don't do it! If you need an extra pair of hands, get someone in as a freelance. If they are good, hold onto them on an "as required" basis. Get them to invoice you for their hours, pay them, and that's that. If they are rubbish, just don't call them again. As soon as you officially employ someone you are into the realms of PAYE, employment contract, NI payments, holiday pay etc etc. Just not worth the hassle.
Also, and I have said this on here many times, try to find a fit (ish) retired person who can fit in with unusual and irregular hours who just wants occasional work. They will work harder, be more flexible, not be on their phones all day and will be happy to work an extra hour or two to see a job finished instead of insisting on stopping at 4pm on the dot.