I lived in Aintree in liverpool when the song was released. There was a lot of speculation at the time about the 'aintree iron', and I don't think the tree was ever mentioned. Most people thought it meant the black bull area (mentioned in the article in the weblink) which is indeed an iron shaped group of buildings (black bull is a pub). I think in the end the conclusion was that the lyric was a bit of nonsense, as no local people had ever heard the phrase before the song was released.