Basic saying goes "measure twice, cut once", and I'm glad I didn't rush through wiring my headlight and tested before soldering & securing the headlight on. As I twisted on the wire nuts, connected the -ve of battery, and powered on the ignition the headlight went on as expect, nice! Then I flicked on the high-beam, bam, high-beam. Then I flicked off the high-beam... I tried flicking the high-beam off again... high-beam is still on.
I doubt that folks at the safety inspection would accept that the latching high-beam is a 'feature', so posted to my favorite forum, addressing some of the very experienced S3 experts on there, here is that account. One of these S3 wizzes suggested that I must have wired the lights in a sort of latch, which absolutely made sense, as did the winging diagrams he sketched to explain it.
The explanation to the latching was that for the two S3 original lights, one of them is wiring straight to the high-beam switch, and the other to the high-beam relay. As I connected the two wires to the single wire of the Monster headlight this created a circuit between the relays input and output, effectively creating a latching relay. The kicker is that Triumph's official workshop manual wiring diagrams are not accurate, but mere rough sketches... good thing for people that really know what they're doing.