A relative is half owner of a house in Yorkshire England. The other relative owns the other half. If one part owner does not want to rent the house can it be stopped? One is there 100%, the other 25%.

I'm in the U.S., so I can't advise you on U.K. law on this. You really need to get a lawyer to sort through this one.
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I suspect the question he or she will ask you right up front is which relative wants to rent out the house? Is it the one who's there 100% of the time, or the one who's there 25% of the time? And by being there 100% of the time, do you mean living there?
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It's an important distinction. If this is the home of one of the two owners, especially if it has been his or her home for several years, the other relative is going to have a hard time forcing the occupant to move out so the house can be rented to someone else.
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If by being there 100% of the time you mean that the house is right across the street from where that relative lives, but across the country from where the other one lives, that's different. It's doubtful that the person in close proximity can frustrate the reasonable desire of the other co-owner to realize rental income from a property that is otherwise available for rent.
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In any event, the more absentee owner is entitled to some benefit his/her ownership interest in the home. If there is a mortgage, who is paying it? Is the resident co-owner paying rent of some sort to the absentee co-owner?
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It's complicated, and if the two parties can't agree on the proper disposition and use of the property, it may be time for one party to offer to buy the other out, or to pursue an action seeking to force such a sale.
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As I said, I'm not familiar enough with U.K. property law to be a reliable source of advice, especially because local laws and practices on questions like this vary so widely. I do suspect that all of the question I've laid out will come up in one form or another, so you might find it useful to think about them before you consult a legal expert.
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Best wishes to you.