From my distant memories of student days and contract law, the dealer is bluffing ( wouldn't be a surprise )

It would certainly not be fair if you could introduce terms to a contract after it had been agreed. If there is no small print, ask them to prove that you had accepted that as a condition of the contract. Otherwise you expect them to fulfill their obligations under a warranty offered as part of your agreement to purchase the car.

I always caution and accept I can be wrong.

You will need to pay for the car to be recovered to the UK.

However, the selling garage is responsible for the fix, and ALL the costs involved. Inform them that it they do not do so, you will get it fixed privately - if need be taking out a loan to get the work done - and will them take them to the Small Claims Court.

Of course, the above assumes they are a proper garage, and not some fly-by-night dodgy bloke who goes through companies like toilet paper

If you are not insured for recovery to the UK, it will be expensive.

Send the dealer a recorded delivery letter stating that he is responsible for the repairs and if he doesnt pay you go to the small claims court.

You are perfictly entitled to mitigate your overall costs by haveing it repaired in the Netherlands.

A warranty is a warranty, the vehicle location doesnt come into it.