Diplomacy often has to be duplicitous in nature: each country needs to safeguard and further its interests against other states. At the same time, governments try to dress up their actions in an accceptable and justifiable manner to their electorate (if a democracy) and other countries.
The current furore over the release of the Lockerbie bomber is rather depressing. It’s unclear what the FCO and the prime minister said or not said to Libya, America or to the Scottish government. However, documents released and leaked tell at best a confused policy line, and at worst double-dealing where the UK government let it be known and promised different sides different things.