Set to 'fan' and 180 degrees, the oven 'overtemperatured' to the extent that I could see a glow of the elements via the fan recess

Your wiring diagram and parts schematics are here in the link below. Normally the bottom element does glow read hot after a couple of minutes. Broil elements last a lot longer because they aren't used as much. There is only one Control Board. If I was guessing, I would say the temperature sensor is at fault. Elements either work, or they don't, but you can easily test continuity with a meter. Many parts are discontinued.

https://www.searspartsdirect.com/model/3f7nkrfnvp-001428/frigidaire-pleb30t8ccb-electric-wall-oven-parts

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Hello Ally;

My name is Peter. I am a retired field service appliance technician.

You may not have a problem. Normally your oven will rise about 40 degrees F. above the set temperature and then shut down, until the oven temperature falls 40 degrees F. below the set temperature, then the oven turns back on.

Your oven temperature is controlled by you oven thermocouple.

You also have a high temperature limit switch, which turns the fan on when you go too far above the set temperature to vent the oven. This may be bad.

Hi Nancy,
180 degrees Celcius is about 360 degrees Fahrenheit.

180 degrees c is equal to 350 degrees f (356 to be precise)

Hello there and than you for choosing Fixya
If your oven does not heat, does not heat enough or does not broil, a common cause is a burned out heating element. There are two elements, the baking element on the floor of the oven and the broiling element on the ceiling of the oven. They are about the diameter of a pencil and typically supported by metal stand-offs.
This easiest test you can do is to turn on the oven and observe the heating element. If it glows red, the element is working. When baking, the broiling element may come on to assist with preheating or to maintain the oven temperature. When set to bake, if the broiler comes on, but the baking element does not, the likely cause is a burned out baking element.
When set to broil, the broil element should glow red, but the bake element typically is not used. If the broiler does not glow, it is likely a burned out heating element.
There are other possible causes of these symptoms, refer to our diagnostic page for some other possibilities.
To test the heating element using a multimeter, follow the steps provided below.

1. Unplug the oven or turn off the power at the breaker or fuse box.
   
2. Follow the heating element back to where it goes through the wall of the oven. Remove the bracket that secures the element in place.
   
3. Unscrew or unfasten any stand-off supports that secure the element to the oven.
   
4. Pull the element part way out to expose the oven's wiring connected the element.
   
5. Label the wires and secure them in place so they do not fall back into the cavity.
   
6. Disconnect the wires from the element.
   
7. Using a multimeter set to x1, touch one probe to each of the element contacts. Expect resistance in the range of 20 to 40 ohms. Infinite or kilo-ohm resistance usually indicates a bad element and it should be replaced.
   
8. If the element tests okay, reconnect the wires, slip them back into the cavity and resecure the element.

Hi guys. Does the Defy Gemini multifuction thermofan oven have a bi-metallic thermal switch? My oven completely switches off around 180 degrees celcius, then resets itself. I dont see a thermal cut-out, so left with the thermostat or timer PCB maybe?

The coils should glow red when working. If the lower (bake) element does not glow it may be burnt out. Some ovens cycle the bake and broil elements during preheat and that mey be why you are getting some heat.

assuming that we are talking about an electric range, lower element not glowing, you can check the element for continuity, froim behind. its either pass or fail. beaware to shut off power wires live all the time

The F2 typically means an overtemperature conditions:
1. over 650 degrees F unlatched or 915 degrees F latched.
2. cooling fan stalled while over above 650 degrees F or an oper thermal switch in the yellow leads.
Check for:
1.Welded relay contacts
2.Cooling fan stalled or blocked air flow to rear of unit.
3.High resitance in over sensor leads/connectors, especially at sensor in rear.

check your fan to see if it is clogged or dirty