If its front then its either a stone stuck in the pad, caliper/pad vibration as Gambit said or the pads have over heated and glazed over. Personally i'd remove the pads give them a good sanding with rough glass paper, clean them and the caliper and then regrease them with lots of copper grease. Also whilst your at it remove the rust lip from the outside edge of the disc with a file.

its impossible for brake dust to build up on a pad

vibration between the pad and caliper is the cause of the high pitch squeal, hence why you use copper grease on the pad backing plates when fitting them

low brake pad indicators will aslo cause a squeel

build up of brake dust on the pads makes them squeel.

there the fronts, the pads are half worn. wot do u mean by glazed over??

front or rears? You sure the pads havent worn down too much? Some pads just squeal, mintex 1144's for example are a pain for it! Try cleaning the drums out if its rears? They could have glazed over which can cause squeal?

wot actually causes my breaks to squeak under breaking?? just gettin fed up of them every time i break now :(
Cheers Mitch