london britain say thursday move toward pay half billion dollars iran debt owe since 1979 islamic revolution nothing bid secure release jail iranian-british aid worker nazanin zaghari-ratcliffe project manager thomson reuters foundation detain april sentence five years iranian court convict plot overthrow clerical establishment deny charge prime minister theresa may government intense pressure secure zaghari-ratcliffe release part foreign secretary boris johnson make remark iran say justify conviction britain seek legal advice whether could transfer 400 million pound 528 million owe islamic republic capital interest 1970s arm deal then-shah iran telegraph newspaper britain owe money shah pay front 1,750 chieftain tank vehicles almost none eventually deliver islamic revolution 1979 topple u.s.-backed leader british officials directly answer request comment whether prepare pay money iran may spokesman say debt issue link attempt convince tehran release zaghari-ratcliffe talk resolve debt matter go years wrong link completely separate debt issue aspect bilateral relationship iran government spokesman say add iran defence ministry remain subject european union sanction debt pay bank account control high court 2002 release due sanction islamic republic spokesman say iran reject media report associate debt britain owe iran fate zaghari-ratcliffe two separate matter ... link wrong nazanin zaghari-ratcliffe try sentence jail tv quote foreign ministry spokesman bahram qasemi say british officials tell telegraph work pay debt intensify recent months bid improve relations iran ask report may spokesman say government saw connection debt fate zaghari-ratcliffe clear see link two issue spokesman say report speculation anything recognize thomson reuters foundation charity organization independent thomson reuters operate independently reuters news britain deny 500 million debt iran link bid free jail aid worker