Geoff the guys have given some good pointers.There is only one thing to do to find out whats going on here and thats to get up there and find out.
You say the house is double glassed and pebble dashed which came first?
I ask this because the dash could be covering the capillary grove on the out side window cill and any rain hitting the window would be directed behind the rendering and onto the brick work trapping it there.
Something else to look at pebble dashing usually has a bell cast above the fist floor window and this can be cracked also trapping moister on the bricks. OK the air brick is a ventilation brick fitted to allow the exchange of air from inside to out and vis a vis.
I have seen seen air bricks fitted in bedrooms but a much better solution would be to to fit vents at the top of each window frame.
CONDENSATION what is it? When cold meets warm this is the dew point and each person releases moister into the air and when this point reaches over load the moister is deposited on a cold surface.
What to do?Insulate the cold surfaces or remove the water?
May be both, if you have cavity walls look into cavity wall insulation and buy a dehumidifier.Any way geoff let us know how you get on and then we will all learn something.Hope this helps billy.