SOLVED. What's the likely cause (and fix) for this window mould please?

Jeepers_Creepers

Fair do's, Ninja, and I'm very pleased that it looks like a solution in on the cards.

Just in case "the gap should be there and air flow is good" comment was aimed at me, can I be very clear that I said no such thing, and nor would I. What I have said, is that the gap under the frame isn't the cause of your daughter's issue - and it isn't. Adding that, ironically, if you kept the trim off, it would prove my point by helping to clear the droplets with the added ventilation!

Just for interest, a failed/misted 'unit' (yes, they are referred to as glazing units as they are double-glazed and not just a single pane) also wouldn't have contributed to your daughter's issue as they are still 'double' and still very effective insulators. The drop in insulation value in a failed unit is marginal.

I'm sure the new window will be a lot better, tho', with perhaps a larger DG gap, and probably a much better frame too giving an improved thermal break.

But I think we all know the actual 'cause', and I very much appreciate you coming back with your honest findings.

ninjaef

Hi @Jeepers_Creepers , thanks for the "final thoughts" . To be fair it looks like the Daughter has pulled one over me for a while. Tempting to believe them when they are working parents and she's nearly in tears trying to "swear the truth". 
Yes, improper drying of clothes, unsuitable venting of the room, poorly fitted frame, definitely irregular if at all cleaning regime, ... need I go on. Many thanks for your insightful posts and sharing your experience and tips. Thank you as ever for being up there in the MSE rankings of helpul forumites.

Jeepers_Creepers

Totally understandable, Ninja, that you were looking for a different cause. And very decent of you to come back with what you'd since found. Tsk, sprogs, eh?!