Well, as it turns out the M Rear spoiler kit is now on (just a stick-on job) and it seems to have stabilised the rear end after all.
Drove it home on my usual commute and nowhere near as nervous giving it the full beans at the usual junctions. Money well spent, says I
And thanks to this forum I might pop over to the boys at MStyle in Essex to see if they can retrofit a front splitter for the next bit. Just because...