Hello, my name is George..
Ensuring firstly that the clock is set to manual cooking.... Does the top element come on when you turn the element onto OVEN setting (not the grill setting)?
If not, then it is likely that the oven is not turning on and therefore it is more likely to be the thermostat control or thermal cut-out that is the problem. Which of these would be at fault I don't know as the parts would need testing with the correct tools to diagnose properly and for this you should call an engineer to assist.
If you need to find an engineer local to you then I can assist, just ask.
If the top element does come on when you select OVEN (not grill), then the base oven element of top oven can be accessed from the rear of the cooker by turning off the power to the appliance and removing the back panel.. The element is screwed in underneath the top oven cavity and is immediately accessible with the back panel of the cooker removed.
Here is a purchase link to the Base Grill Oven Element
When you get to the Checkout page.. To the left of the 'Pay by Card' button there is a link to allow you to enter a discount code.. Click on it and enter the discount code __ id1975 __ to get a discount
If you need further advice on fitting this part I can assist.