I was waiting to get to speak to him today. Which I have now done. I am sorry for the guess work, but I originally posted in order to get a better understanding and what sorts of questions to ask, and to undersrtand the answers, and implications of the situation ahead of the conversation. And I was far better able to have that discussion as a result so thank you to you and others who have contributed.
I did not state where I am more specifically as I am conscious that my understanding is poor and I don't want to do anything that might identify the person if I have unintentionally misrepresented things. Plus I am not sure that was important to the question I posted.
He has confirmed that he is not registered, or part of a scheme or body, though has qualifications. He has said membership of a competent person shceme is something he has been planning to/thinking about doing and he has offered to get an NIC inspection of the work, if we let him continue, as part of his membership app. So we are considering this and will decide whether to proceed on that basis with assurances on other points raised here e.g. insurance.
So again thanks to all for your valuable help and especially for the wealth of straightforward non-technical advice.