I mounted a new set of winter tyres (Bridgestone Blizzak LM25) on my original Quasar alloy wheels. During the first motorway trip, the steering wheel started to vibrate after 115 km/h. Evidently a balancing problem.
I came back to the garage with the complaint, and the guy who balances the wheels told me that he can't do much, although they have the famous Hunter GSP9700 machine, he cannot mount my wheels on it so he had to balance them with the older, more basic machine.
The problem is that Quasar wheels have a very small hole in the center, and the axle of the machine just does not fit.
 
Anyway, he tried to re-balance the wheels with the basic machine, but no luck - although he put serious effort to it, the car still vibrates.    
 
Just for completeness, the car went great prior to the change (summer tires are Goodyear NCT 5)
 
Please help!

Ton garagiste doit utiliser un centreur court sur l'équilibreuse. (comme si le centre n'était pas débouchant).
Ca existe sur toutes les machines.

:coucou  
S'il n'a pas l'équipement pour adapter les jantes Quasar à son équilibreuse, il peut (il doit)  confier cette opération d'équilibrage à un confrère mieux outillé.

Thanks for your responses.
 
I left the car for a day in the garage for them to have a thorough look. They have rotated tyres on some wheels and had 2 test drives, with static balancing corrections between them.
The original vibration of the steering wheel is gone, but the car feels funny in the seats.
I shall wait until spring when I change back to original tyres.