Thanks Keith, that's exactly what I was hoping to hear.
One thing I didn't mention, my wife is not British (or from the EU), but she has lived here for around 20 years, has her own NI number and has worked a number of the years in the past and made NI contributions (she has a a "Given leave to remain in the UK for an indefinite period" stamp on her passport issued by the Secretary of State for the Home Office).
I presume she can set up as a sole trader, which seems to be the most simple option??
Thanks again for your advice.
My wife is a Thai with exactly that stamp in her passport! If you look at the paper work sent to her on her first entry to the UK you will find an official letter to the effect that she may undertake any employment save that which is precluded by other regulations eg say as a medical practitioner, dentist, vet, barrister or solicitor.