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If your father simply allowed them to park the car on it then he has not transferred it to them – it remains in your father’s ownership.
However, if he has allowed them to park there for 30 years continuously then the previous owner could argue that he has acquired a right of parking by prescription. This is another way of saying that they could argue they have acquired the right of way legally by long use.
However, if the previous owner did not register the right of way at land registry (you would have been told by land registry if he had) then it may be difficult for the new neighbours to enforce because they would need the previous owners testimony.
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