Firstly the back calculation IS legal. You cannot dispute this now as you only have 21 days in which to appeal against a conviction in the magistrates court.
Elephant insurance is a part of the Admiral Group who are notorious for having The clause in their policies that not only will they refuse to pay out for your own vehicle if you are over the drink drive limit (almost all insurance companies do this) but also that they will seek to recover the costs that they are obliged to pay out to third parties as a result of the accident. They are really saying that because you were drink driving you were negligent, and they only cover accidents which are ‘accidental’.
They have 6 years to bring proceedings for civil cases and I would urge you to seek legal advice about this. Given the length of time already elapsed then the company may well issue what is called a “Letter before Claim” which is the final step before they commence proceedings. If you do not respond to that, they will issue a claim in the civil court. Because of the size of the claim, it would not be heard in the small claims court so if the case was proved, you would also be liable for their solicitors costs.
Having read their claim letter, the solicitors costs at +£11,000 seem high, and there are NHS costs for +£6,000....? Perhaps the person injured was not a UK national?
You should get a solicitor to challenge the total amount and justify that they have been diligent in keeping the claim costs to a minimum, did they seek any medical opinion if the injuries were as described? They have added about £1,000 for dealing with the claim, and £400 for their solicitors, against £11,000 for the claimants solicitor. It may well be that the figure can be negotiated down but if they do take you to court for this you could face selling your house or being made bankrupt to pay for this.