This does sound like you need to get a regulatory body involved.Well I think NFU are clearly out of their depth. Next month will be into Jeremy's fifth month and the vehicle not fixed or replaced. JUST NOT GOOD ENOUGH!
Most people would have to hire a replacement at a huge cost and this is normally factored into the insurance repair cost.
The last picture you posted shows the vehicle partially stripped and open to the elements. I would start with citizens advice then a free consultation with a solicitor, maybe pay for a strongly worded letter.
I would be tolerant and helpful until a point and I think it's would have passed it by now