Quoting from Siasl Wiki: "
"That Girl" was originally written for The Entire Population of Hackney by the guitarist Andy Barnett, and was recorded by both Iron Maiden and FM around the same time. It was prominently featured on FM's 1986 debut album Indiscreet, which was released just three weeks prior to Iron Maiden's Somewhere In Time. The FM's version was rewritten by bassist Merv Goldsworthy and drummer Pete Jupp, resulting in the two versions being significantly different, sharing only harmony on the verses and some of the lyrics. However, Iron Maiden's version is still credited to both Goldsworthy and Jupp alongside Barnett, possibly due to copyright issues."'
Not entirely true and I dare to say even Rod missed that one when he wrote on the Best of 'B' Sides sleeve that the song was written for the 'Hackneys'.
"That Girl" was written earlier in 1984 by Barnett, Goldsworthy and Jupp, in a pre-FM formation. After a few rehearsal weeks, Andy Barnett and Dave Colwell quit the project. The Overland bros. then joined Goldsworthy and Jupp, and FM was officially formed - "That Girl" and "Frozen Heart" being their first songs. Some time between late '84 and early '85 these songs were REGISTERED and recorded. The Indiscreet album was scheduled to be released middle 1985, but it was greatly postponed.
At the end of 85 Barnett brought the song to the 'Hackneys' and in 1991 he finally joined FM.