Webby, how come you aren't sat in a nice shiny new Hire Car paid for by the third party insurers? 
After a small amount of digging, the general information I can find is the buy-back would be 10% of the car's value (or I should say 10% of the offer made by the Insurance Company) The value and offer should tally-ish.
Cat D write-off, like yours would be (I erroneously said Cat C above) (if it is indeed written off) doesn't need a VIC test or anything, just stick a bumper and tail-light in and drive it away.
On my first point, push push push for a hire car from them.... That's a costly thing to them and you are more likely to have a speedy conclusion to this mess if they are paying £30 a day for you to spend around in a lovely Peugeot 107 
Belatedly sorry to hear of this sad news Webby. I do hope you sort everything out to YOUR satisfaction. As others have said, take no nonsense; it was not your fault so you must not lose out in any way. BM is right - you should by now have a good quality hire car at your disposal.
Over the last 3 years I have had two "non-fault" accidents. On both occasions I had a EuroCar Mercedes E class, a 320, at my disposal for weeks whilst they decided what to do with my cars - one was a write off, the other they tried to write off but I refused to allow them to do so - and both times the third parties insurance had to cover the hire car costs whilst I sorted out a replacement (which cost more than the w/o car was actually worth) that I insisted had to be at least as good as my existing car, and in the other case until another repair centre, not the one that tried to write it off, made the car good.
In both cases I also received all costs incurred, and my excess was completely covered by the insurance companies agreeing on a direct payment from one to another without me being involved.
This was the write off:
far worse than yours which should be repairable
and this one they tried to write off but finally agreed to repair:

Cannot see the damage? No, there is just some light damage to the paintwork, but the buggers tried to write it off, and I drove to the car repair firm immediately following their phone call and grabbed my car back. Proves what others have said is right!
Everything should be to your satisfaction!