When you say the top cooker not working I take it you mean the all the hobs on top?
All the hobs are working, just the top oven and grill are not.
Ok when you turn the top oven dial on does the little light on the front light up?
No the on light does not come on.
Ok if the top oven or the grill isn't working then its possibley the thermal cut out at fault on the back.
What you need to do is remove the back panel and look on the back of the top oven cavity and on the left hand side will be a bracket with the thermal cut out on it. Its round and will have 2 wires going to it.
Check the wiring and if you have a multi meter you need to test this for continuity as well and if faulty then you need to replace it.
But if that's ok then you may find the top oven thermostat on the front that you turn is the fault here.
But check the back thermal cut out as if both the grill and top oven don't work its more then likely this at fault.