Hope I'm not going to regret this.
The best thing I found for cleaning Alcantara [or anything] is brake cleaner.
Squirt some on to a clean tissue/kitchen roll, apply to material, allow to soak in for 2-3 seconds then soak out with clean tissue.
Do not rub or allow to dry without soaking out as much as possible.
My local garage tells me it is actually dry cleaning fluid
I haven't researched this, i'ts what I've been told
Now I just know someone will let us know correctly.
Bob S.