Is there any reason you think you may not?
Well, surely it is unexpected? This was the contract price?
This is their mistake of course. It is not your fault that they made this mistake. That doesn't mean you don't have to pay though.
You are liable to pay the contract price. I realise they made a mistake but that doesn't remove your obligations under the contract.
Sorry if that is bad news.
Can I clarify anything for you?
Yes, there is.
It is six years.