Sorry Biggsy, I was doing everything right with the M+'s, AFAIK, & they will be sold on. It was a case of the beads not sitting right on the rim and no matter how hard I tried, they just wouldn't go on until BANG!
The Marathon Extremes look like the right profile, but I can't afford £70 for a pair right now (am on the dole), so I'll take the (much) cheaper option IF it works.
FWIW, my nemesis recently has purely been the hawthorns, so if the Slime tubes will seal that kind of puncture well, it's a go.