These accusations stalled Warner's career briefly: he remained in London for five years before in 1956 he was made acting Chargé d'affaires at the embassy in Rangoon, Burma.
Despite Warner's unstuffiness (which ruffled some feathers among the Japanese civil servants who expected more formality), he became popular and was awarded the Order of the Rising Sun, 1st Class.
He was disappointed when others were named to these posts, and Warner retired from the Diplomatic Service in 1975, reasoning that it would be easier for him to start a second career as early as possible.
He specialised in international affairs, successfully urging the European Commission to help fund Afghan refugees and bemoaning the Iranian government's failure to respect diplomatic immunity in the Iran hostage crisis.
Standing down at the 1984 election, Warner co-authored a pamphlet that November calling for the United Kingdom to become a full member of the European Monetary System.