It's not really about the internal temperature, that will only have a small effect. The temperature in a cold house will still be much above the outside temperature at the cold times.
The key thing is the air seal on the double/secondary glazing. It is denying moisture from being able to migrate into the window as condensation starts to form. There is not enough moisture in a sealed double glazed window for condensation to form but when the seal fails that's when it does.
Sorry mate, but that's just not the complete picture there. Condensation will form most where you've got a cold surface and heavily moisture laden air. The internal temperature of the room has a significant effect on the temperature of the internal most pane of glass.
Not all double glazing is filled with fully sealed air cavities, or a vacuum, or an inert gas etc. Yes you can get condensation forming within the cavity, but the air most laden with moisture is going to be the air in the bedroom, more so than the air in the cavity.
Reducing moisture on windows specifically revolves around having the temperature of the inside pane of glass sufficiently warm enough as not to condense the moisture in the air upon contact with it. You achieve this by heating the room, which could quite easily be 10 - 16 degrees in winter if not heated. (For example a lot of people don't heat the room at night, but instead use an electric blanket). In this case, you will get condensation on the windows, regardless of whether they are double glazed or not, as you have done nothing to heat the glass.
Of course, removing the moisture in the first place will also help, but my comments referred specifically to DIY double glazing.
"Don't expect unheated spare bedrooms to be condensation free just because they have double glazing."
That's what you said and it's wrong to my mind. The inner surface (to the room) won't get condensation, why?, because it is too warm as the air in the gap will be about of the average of inside and outside temperature. Condensation may form on the inside of the outer pane as the outer pane is cold (on the inside as well), that condensation must come from the air inside the room and the degree to which air can move from the room to inside the window is the key. Of course, secondary glazing that doesn't have a seal is effective to a point but that is because it creates still air that doesn't exchange with the room so readily.