the hard facts are that you did hit the other motorist and you ar eliable for the accident. if other driver puts in a claim for injury ,this muct be backed up by medical evidence. Your insurer will study this evidence and will come to a view on the value of the claim. If claimant has gone through PIAB, then then the case is probably being treated as an assessment by your insurers and the award made by PIAB has the same validity as a court order, so they must pay out.
Difficult as it sounds, a payment will be made and there is nothing you can do about it.
your only option, and this only if the insurer agrees, is to meet insurer ask them how much they think the case will cost; ask them to pay you that amount and in return ,you fight the case through the courts at your own expense. You will probaly have to agree to lodge some substantial funds with insurer if you intend to do this,.
If at the end of the day, claimant gets less, then you have the moral satisfaction of phyrric victory, if he gets more, then you are on the road to quick bankruptcy.