This advise assumes you have electronical knowledge & qulifications if not consult an electrician.
Will not be held responsible for bad advise or bad spelling
I have the same problem you need to remove back panell then remove 4 screws from the upper part of end brackets (2 each side). then after you have removed all hob parts you must take out all small cross head screws from around each gas burner, then the top can be removed. Now you will see the selector switch that when operated from front knob should energise the various elements in different combinations
with multimeter check outputs, you may find that the contacts inside selector switch are burnt out in certain combinations with loss of supply to certain eliments. If so replace this switch. On my oven the circular fan element will not work on full oven setting but will work on fan assisted grill. have not found who supplies parts yet but this is where I have found fault.
Hope this is of help.
Steve D
December 2010