So, I eventually got to the bottom of why I couldn't insert my spring compression tool into the o/s rubber cone, and I'm surprised that some of those Mini guru's out there didn't pinpoint the problem. I thought the cones were original, but it turns out they are not, the reason; the o/s rubber cone has an imperial thread, the others are metric! At some stage the cone(s) have been changed and for some bizarre reason someone put a random imperial rubber cone in for good measure... Nice!
Ah, OK.
The early threads were Imperial, but they were phased out about 1974 or there abouts ! There wouldn't be many left in cars these days, they usually get changed out as just with age, they not only sag, but get hard (and can get very hard).
As far as I'm aware, all the new cones, genuine and after market since the 70's have been metric. Given this, it's not something many here would have picked up on !
Oh well,, one for the 'experience' books !
I have a 14 x 2.0 mm long tap. I run that through every cone - old or new (even brand new) - before I fit my compressor to it. Then if it's imperial, that's sorted and also, if the thread isn't clean, it sorts it before ruining the thread on the tool.