You're shooting your own argument in the foot here

"And yes, before anyone mentions alcohol - if it was discovered tommorow it would be Class A within a week and with good reason."

Yes. And we would have another banned substance, inexpensive and easy to transport, which would cause another lost battle in the War Against Drugs, which would in turn harm the society much more than alcohol itself, by giving loads of money to criminal gangs and law enforcement agencies, and putting at risk the lives of consumers and the general public.

It has happened before, you know. The Prohibition and the ban on cannabis are pretty good examples, but not, by far, the only ones.

"And you can't legalise something on the basis that people are only permitted to use it at home and not drive their car or go out for a day afterwards - it won't happen"

If breathalysers work, the same concept should work for ant other drug and any period of time. Set a maximum limit of drug in the bloodstream or in the breath, choose a testing method and use it. Not such a big deal.