Someone on Twitter just helped me out... the track is:
Maggie Gave a Thistle by Wayne Shrapnel and Oi Stars
Odd thing is, there's no mention of that track or 'Wayne' anywhere on the internet is there? Most searches lead nowhere, and some actually lead to the This Is England soundtrack CD... which it isn't on.
But... conspiracy theorists pay attention. It seem that it was originally on the track list for the CD, but it wasn't included by the time the Cd was released.
Yeah that's strange. If it is an old track from an obscure, pre-internet days band, presumably somebody cleared rights for it to be on the trailer. Maybe it's a pseudonym? Be interesting to hear if anybody know.