The A-side was written by Labi Siffre. His version, titled "A Little More Line", was included on his excellent debut album on Pye International NSPL 28135. Images of this album on the Vinyl Albums site.
Decca has a matrix ZDR 56312, but it's the B side of Decca F 13552, from as late as September 1974.
Additionally, ZDR 56312 is on both sides on this 45 we have here, so it is supposed to be a catalogue number and not the matrix number. Further, ZDR would mean a stereo recording made in Decca's own main studios in West Hampstead, which this murky mono recording definitely isn't. A mono tape coming from outside Decca would have been XDR, or ZXDR if in stereo.
So it is probably pure coincidence. Or maybe K. J. Roberts had seen some matrix numbers on Decca's labels and decided that it would be cool to have a similar, however meaningless, number on his record...?