Until 1991 Yugoslavia was a Socialist Federal Republic comprising the Republics of Serbia, Croatia, Macedonia, Montenegro, Slovenia and Bosnia-Herzegovina, as well as the autonomous Serbian provinces of Kosovo and Vojvodina.
In mid-1991, Slovenia and Croatia disassociated themselves from the Federation.
In April 1992 the Federal Republic of Yugoslavia, comprising Serbia and Montenegro, was established but in June 2006 Serbia and Montenegro became independent states and the state union of Serbia and Montenegro ceased to exist. In respect of Serbia, the 1982 Double Taxation Convention between the United Kingdom and Yugoslavia is regarded as having remained in force.
The text of the Tax Treaty can be found via https://www.gov.uk/government/publications/serbia-tax-treaties