Is it a bathroom Wendy old boy? And how old is the property? If it's an old property, there may be an air brick that's been plastered over internally, have a look at the outside wall, see if there is one in the right area externally that's been covered over inside. It could always be opened up again if so.
If the window is double glazed and condensation appears between the two panes then the seal has indeed gone. A new double glazed unit to suit shouldn't be exhorbitant.
If the owner can bear to keep the window open a tad this will help stop the damp marks on the ceiling. Also installing an extractor fan will do the world of good. One with a humidstat would kick in when there is steam present and shut off once it's cleared. It's not such a major job to install one....can't be, thinking about it, it's one of the jobs I do for a living.

The property is late 80's (no airbricks) in a bedroom just around the top of the ceiling above the window which has the condensation which only started in last few weeks. I think the condensation does appear between the panels but it is all over the insides of the window especially when waking up in the morning, which has coincided with the damp appearing.
How much is installing an extractor fan?