IME the 18k will quickly be dwarfed by the total bill with airplane/ferry tickets, hotels, lawyer time, etc.
This is where it lies, legal costs are very high in Ireland, the costs will be even higher as across border.
Customer wants Extra Work, Specify Requirements and Quote for work, Receive PO from Customer (or written acceptance if small company dealing with boss), Begin Work.
I am in the same country as the OP, a former boss was not shy on taking court cases.

Our standard contract was written by the person who wrote the main text book book on writing technical contracts! Most clients who did not look at it in detail, but the odd big one with in house legal team did, and had a fit

They were told to take it or leave it. For 18K the OP would need to forget about the courts.
The main question is, legally, does he have any ground to stand on about breach of contract?
The PO is not on strong ground as he never fulfilled the original contract. It will cost a multiple of 18K to bring to court, take years and cause more stress than writing off the money. If you lose a court case you could be out of pocket by +100k.
And do I have any obligation to do any more work to deliver the original specs too?
You signed a contract. But the reality is there is not much the client can do, unless he has deep pockets.