I started my own business taking on employees for the first time last year I registered for info at www.hmrc.gov.uk/paye/intro-basics.htm This is my take/ opinion/ interpretation...
You can pay cash, cheque, bank transfer etc thats fine but you do need to keep records of who you've paid what.
For each person you 'employ' you need a P45(thats the bit of paper they get when they finish work stating what there tax code is) or you get them to sign a P46(thats where they self declare they have worked this tax year, they haven't worked this tax year and don't have another job or they do also have another job). I have always needed P46s to date its really quick and you either start them of with a full tax allowance for most casual stuff this means you don't have to subtract tax or NI you just do a year end return, which didn't take long at all doing it myself for the first time online.
If they're not working for anyone else you're not taking anything off them if the payments are less than about £100/ wk so you basically are getting them to sign a P46 and giving them a reciept (payslip, which can be hand written so long as it states a few basics details on above website) - copy for your records then once a year filling in who you've paid what
You need to verify they are entitled to work in the UK so you need an NI number, passport, etc theres a list of officialy acceptable docs you can check out via the above link. You need to do this will all employees otherwise you risk people claiming racial abuse.
I struggle a bit with the employ people and cash in hand. If people are willing to do a P46 the paperwork is really minimal and you only need to pay in tax deductions 4 times a year if its under £1500/ quarter your deducting - which I guess it would be. I do my returns online and it takes me about 1/2 an hour every 3 months and about an hour for the year end return. If you employ more that 5 people at one time I think you need to have employers liability insurance. Mines from AXA and costs about £170/ yr - which I guess would vary with the type of work you're in.
The advise lines open long hours and the online element of the website is quite good once you've got your head around what it is you need from it. The inland revenue do run free courses from time to time about setting up a ltd company, employing someone for the first time.
I would suggest you phone and request a new employers pack this will help make more sense of it all - it did for me.
Its all a bit daunting at first its like you need to know what to ask specifically to be able to get the answer to what you need to know! So many new phrases and form codes to make sense of.
Hope this helps