I guess the forensics folks have proven it was magnesium. Can't believe that they couldn't. Or lie about it.
As for proving it was Russia that's a totally different matter. Maybe they can determine where the package originated, but not obvious to me how they can determine that Russia placed the magnesium into the "massage appliance". A few similarities to the pagers etc. in Lebanon perhaps.
Do I trust MI5 with their analysis? Perhaps, perhaps not. I just think back to the Iraq debacle under Tony Blair, and the BS that underpinned that disgrace. So, I'll stay on the fence this time.
As for incendiary massage devices, I saw plenty of them recently on some stalls near Sukhumvit Soi 4...