I added a bit of self amalgamating tape under the mouthpiece & tightened the cable tie up a bit more that previously. Also reseated the two plastic cir-clips inside of the flappers on the BOV.
Well that didn't work out so good! In fact I made it very much worse, to the point of being completely unusable with obvious water getting in around the mouthpiece. I'll have another go, and check my work before I waste another day's diving...