Astra brake clonk?

Can anybody tell me why the front brakes on my wifes Astra Bertone clonk when the pedal is pressed they are fairly new discs and pads (3 months old),which were fitted by the trader i bought the car from so i suspect they may be a cheap set i recently cured a similar noise by altering the retaining springs on the back of the pads where they locate in the piston of the caliper the performance of the brakes is also poor i have checked and changed a faulty rear wheel cyclinder which improved things a bit but they still don't pull up as well as they should will fitting a decent brand name set of pads improve things?Any help much appriated.