I have never come across winter tyres (M+S) ever being run at lower pressures than summer tyres.
From another Forum/Blog... follows
Other myths about tire inflation include keeping the pressure lower in winter than in summer; and that the best time to check tire pressure is after a long trip.
In fact, tire pressure usually lowers itself in winter and raises itself in summer-it requires no action on the part of the motorist apart from keeping the pressure at the level recommended on the sticker that is usually located just inside the door, glove box, or fuel door. Under normal loads, inflate tires according to the vehicle manufacturer's recommendations, NOT the maximum pressure listed on the sidewall. In any case, one should never exceed this maximum pressure.
The sidewall, incidentally, also shows the type of cord and number of plies in the sidewall and under the tread.
Every time the outside temperature drops 10 degrees Fahrenheit, the air pressure inside your tires goes down about one or two psi. Drivers should check their tire pressures frequently during cold weather, adding enough air to keep them at recommended levels of inflation at all times.