Hello,
That sounds like half a rip-off to me. Assuming the repairs were properly completed and the car is in genuinely it is worth £1,000 - £1,400.
The buy-back price seems OK though. Buying the car back and having it repaired, is usually the best thing to do in these situations.
You need to make a formal complaint, which the company has 8 weeks to deal with. If they have still not made a sensible offer then you should refer it to the Financial Ombudsman Service on 0845 080 1800.
If it were me, I would ring the insurer up, with a copy of Auto Trader (and perhaps a Used Car Expert Price Guide!) and ask to be put through to a supervisor.
I would politely give them examples of the price in the guide and of cars for sale. If they don’t budge…. I would scream at them until they agree to a fair payout.