Paying someone on a non-regular basis

[XLMonkey]

Does this mean that if a geezer from down the road fitted a bathroom for you cash-in-hand, and Hector caught up with him, you'd be liable for his tax & NI?

Not disputing your argument - just intrigued.

Yes, it does. This is the reason that most companies/agents won't deal with contractors who are registered self-employed: if you don't pay your tax, then HMRC can pursue them for it instead.

On the OP's question: the options are either to deduct Income tax and NI at source (so treat them as an employee, albeit one employed on hourly rate rather than a salary); or to get them to confirm that they are registered self-employed and pay them gross.

In the second option you still have the risk that you could get chased by HMRC for any unpaid tax, but it depends on how great you think that risk is (they are unlikely to chase you for a few hundred quid)