On Swedish copyright rules: see https://www.stim.se/sv/om-upphovsratt
Generally, the copyright lasts for 70 years after the composer's death, in this case 1976. STIM (Swedish Composers' International Music Agency) is on Facebook, https://sv-se.facebook.com/stimsweden
Their phone number is +46 8 783 88 00
Hope this is of some help, Mg. Best of luck to you! "Blue Bird of Hull" is one of the most gripping ballads ever written in the Swedish language.