First of all, you have to keep in mind that the commercially available version of It’s Grim Up North was a very polished remix of the original version that had been released in December 1990 and which was clearly influenced by their Pure Trance series. They dropped Pete Wylie’s vocals and had Bill Drummond re-record them and added the orchestral outro ‚Jerusalem On The Moors’ to the end of it.
Ok, that’s what happened to most of the Pure Trance singles as well which were part of the Stadium House period of The KLF. But why did they credit it to The JAMS then – who were apparently dead after their second LP? As we all know, It’s Grim Up North was meant to be one of the tracks of the ever-rescheduled ‚Black Room’ album which went through various different styles. In 1991, on a norwegian radio show Bomlagadafshipoing (really!), Bill indeed talks about the recording of the track as a part of the upcoming album.
Jimmy went to a club, a rave, once, last November, and the PA had broken down. It was like… It wasn’t very good. And all the walls were concrete. And there’s this sound to all the records, a very horrible, hard sound. And we wanted to get some of that. And we… I mean… we had decided we not wanting have snares. All these little things. Just, just. The only drum sound is the bass drum sound. And everything else is almost just noise. That’s what we wanted on the Black Room. […] [It’s Grim Up North] was the first track when we first decided to do The Black Room album. It was the first track we did for the album. We hope to redo that track and make it commercially available. And then it’ll be the main track on the The Black Room album.
This leads to the conclusion that they had already decided to release ‚The Black Room’ as the Justified Ancients Of Mu Mu when they first came up with it, so It’s Grim Up North naturally got credited to the JAMS as well.