HOPES that Cyprus will be able to join the European Union as a unified island on May 1 have suffered a setback as negotiations over a proposed referendum have run into serious difficulties. Talks under United Nations auspices, due to be concluded in Geneva this week, have been put in doubt by the reluctance of the Greek Cypriots to agree to the terms of a referendum, or even to the holding of a referendum on the previously agreed date of April 20. For years the Turkish Cypriots have been considered to be the more opposed to any kind of reunification of the island's two separated communities but now the Greek Cypriots appear to be the ones making difficulties. In one sense they have little to lose because if no agreement is reached the Greek Cypriot section of Cyprus will officially become the European Union member while the Turkish Cypriots will be left out in the economic cold. However, there are wider issues at stake since Turkey itself is extremely anxious to become a full member of the European Union as soon as possible and does not want to be accused of failing to persuade the Turkish Cypriots to opt for EU membership. When talks were held in New York last month there appeared to be an agreement that if the current negotiations failed the United Nations would step in and itself hold the referendums in the two communities on whether they wanted joint membership of the EU. It now seems possible that the Greek Cypriot leadership will not honour this agreement and instead refuse to hold a referendum among its people. At this late stage only the strongest pressure from Athens and Ankara can save the situation.