Good morning Emma.
I need to ask a few questions before I can give you a confirmed answer,
1st when you turn the thermostat does the oven light should come on and the fan motor should be running, could you please confirm that for me.
The 2nd one is a technical question do not attempt this unless you are a qualified electrician,( most people know an electrician that they could butter up)
You need to check the thermostat has 240 volts going into it I will enclose a pictureof one.
if there is no voltage going to the thermostat then this fault could be the time or a cutout switch
if there is Voltage going into the thermostat but not coming out when set to a temperature then this looks like it's the thermostat,
we will be happy to help on the part when we know which it is paul