My mate bought a car back from his insurer after they wrote it off. Classed it as Cat D. He got it repaired, damage was rear end with boot area only damage. He got it repaired significantly cheaper than the insurers quoted cost. His mate/repairer gave him a quote and he offered the insurer a sum taking that into account. They accepted his offer. It was an Escort and in very good condition but age and market value influenced the write off decision. The repair was very good.
The documentation, who, how the repair was done are immensely important to the resale. For that Plus8 it would be interesting to see what answers are given to questions about the history.
“Has the car had any accident damage? “ is one I would ask. The answer would be crucial, especially if the car was known as CatD. Then the haggling would commence!