I hate to admit it Julie but many moons ago I worked for Zenith for a while (until I left in disgust). It's a myth that double glazing stops condensation. The only way to stop condensation is adequate ventilation which can only be achieved by opening the windows. Opening the trickle vents helps but that's just another way of bringing fresh air into the room. The average person thinks that double glazing eliminates condensation. Although it doesn't we were always taught that whenever a customer told us that as they often did, we were just to smile and keep quiet! Double glazing only reduces the amount of water that forms on the glass compared to a single glazed window.
Condensation forms because of the difference in temperature between the air in the room and that outside. The glass will be cooled by the outside air and that enables the moisture in the room to condense on the glass.Your breathing keeps the humidity high in the bedroom at night, especially if you have the door closed.
There are two ways to stop your bedroom window from forming condensation in cold weather. Either open the window at night or keep the bedroom door open and open other windows in the house to provide a flow of air. If you don't want to do that then you will have to put up with the condensation.