In my opinion the car is easily worth £20,000. Mine has an agreed insurance value of £10,000 and that figure is just based on the receipts for the parts I have bought it does not include any labour for the 2 and a half years spent building it. Marks car was finished to a much higher standard than mine so should command a much higher price. There are probably only 2 or 3 Nova's that are to this standard in the UK. So I would say very rare in this condition.
Dirk