If you set it to fan assist mode will the fan inside the oven run?
Does the little light on the front panel come on when you turn the thermostat on?
Ok does it get hot on fan assist mode?
Ok if you set it to conventional oven setting so it uses the outer part of the grill and the base element does this get hot?
Ok strange fault as its not the thermostat or the thermal cut out on the back as if they were faulty it will stop the heat on all other modes.
The timer doesn't control the top element and as you have replaced the element and selector switch it leaves nothing left. The only other thing it could be is a wiring fault.
Have you checked to see what voltage you get to the wires when the gill is selected.
Test each wire if the grill element to earth and see if the wires have 240v or not.
Let me know what you find.
No worries just let me know as there should be a live and neutral for it to work so if all the wires are 240v then you have lost the neutral feed to the element.