What is the fine-grained topic of the following text: SDLP Foyle MP Mark Durkan has said the huge leak of confidential documents from Panamanian law firm Mossack Fonseca should be a wake-up call for the British government to finally call time on the rich and powerful using tax havens to hide their wealth. Only return the topic.
Mark Durkan's call for British government action on tax havens in wake of Mossack Fonseca leak