To see the lyrics, click on the name of the version you are interested in (on the left).

Tring to understand what the song is about, all we can gather is that it is about war (World War II ?) and that it takes place in the London suburbs. Osterley, which is one, is mentioned. Metroland (also written Metro-land) was an area of England served by the Metroland line. This line started from Baker Street, in London. During World War II, the army’s headquarters were located in Metroland.
Even though Alan Clark could be the British conservative, who actually wrote books on the Second World War, it is also very likely it refers to Alan D. Clark, an American officer of the US Army Air Force (during that same war) involved in the Roswell case in 1947.