Distributing inheritance

I've been mandated by the family to be their General Power of Attorney in the handling of the properties of our late father. The property in question is in Pakistan. My late brother, an heir, was a resident in the UK. I requested documents from his siblings in the UK, namely a General Power of Attorney, which they obliged with. However, the documents needed to be attested with the Foreign Office and the Pakistani Embassy in the UK. They have had the documents attested, but now they are refusing to send the documents back to me. Regrettably I've sent a letter of demand/return of documents to them, with no success. What should I do?