Your theory about the Sonoton release could be right, Malcyb. I thought I'd take a listen to other KPM's released around the same time. KPMs 1050 and 1052 are both in mono. However, I instantly recognised KPM 1052 (Christmas Party) as being identical to the MFP release of the Torero Band's Tijuana Christmas. The MFP release is in stereo. So it could well be that, similarly, the original recording of 1051 was made in stereo but that KPM released it in mono. There were only a couple of FM radio stations broadcasting in stereo in the late 60s (BBC Radio 2 and 3 if I remember rightly) but all other radio stations in the UK were transmitting in mono as were all three terrestrial TV channels. So that may have been the reason for the mono release.