Complex sentence: In 2014, UNESCO had offered to mediate between Greece and the United Kingdom in resolving the dispute of the Elgin Marbles, although this was later turned down by the British Museum as UNESCO works with government bodies, not trustees of museums.
Split into simpler sentences: In 2014, UNESCO had offered to mediate between Greece and the United Kingdom in resolving the dispute of the Elgin Marbles. The British Museum had to decline the offer as UNESCO works with government bodies, not trustees of museums.