An update on this.
I had an mri scan and then saw a spine surgeon recommended by a consultant I know.
He was fascinating, spent way over my time chatting and asking questions, and concluded by telling me that my spine is pretty worn out but there is nothing operable, but given what he'd found out about me he suggested that I speak to a therapist.
He said that 'pain is a powerful emotion' and that my emotional battery sounded pretty flat. Not what I expected at all.
I have found a therapist with one spare slot (obviously most are totally booked up) and after a few sessions I'm already able to sit better. Seems she thinks I should write a book given my family history and we've hardly started on my recent past..