Hi Kev,
Sorry to hear your bad luck on this, personal favourite car is the P1. We recently had our other Impreza written off after being rear-ended while parked, caused £8000 damage to a £7450 valued car that we had insured at it's value of £9500. After a long and embroiled battle with the insurance company we managed to prove that it could not be replaced for the money they originally offered in settlement. In order to prove it we had to produce current adverts relating to the sale of a similar make, model, age, mileage and spec to the car being written off. It was also said that the cars had to be for sale within a 100 mile radius but on something as rare as yours I'm not sure this would apply.
Given how much you insured yours for and it's probable condition and mileage, I would argue the case strongly that you cannot replace it for the settlement being offered. Settlement on a claim like this has to enable you to purchase the same specification, make and model of car again in replacement for the one you have lost.
Good luck and fight for it!
Peter