Thanks Gareth for your helpful reply......although I did read your first paragraph several times and couldn't quite understand the reference to 'cats', thinking you were referring to catalytic converters!
My thoughts on the steering problem so far are:
Earth fault
Water ingress in wiring connectors
Defective sensor
Software requires update
I had thought of reading fault codes to see if that tells me anything, but intermittent faults don't always show up and the dealership technicians are often at a loss to explain the problem.
Kind regards,
Barry