Default steering wheel shaking and wobble

hi all,
im at my wits end with my 11 plate sri nav auto insignia. had a new set of disks and pads less than a thousand miles ago and since have had a constant issue with the steering wheel shaking and wobbling at all speeds but worse on the motorway. had the car in my local dealer today and they said the disk and pads needed replacing as this was causing the problem! all lies as the work has been done but the problem is still their have had thee wheels re balanced and the tracking checked and no different can anyone shead some light on this for me please?

regards

anth