No problem Dave, and welcome to Briskoda, by the way.
Your Fabia is fairly chock-a-block with control units and it's not really the case that the engine ECU is the main brain, just the main engine control. The interior light, door locking and even oil level monitoring are not within its scope of operation. Oil level (and temperature) data are processed by the instrument cluster itself, which is a control unit in its own right.
If you're handy with a soldering iron, reflowing the solder joints of all the connector pins from the two connectors on the instrument cluster circuit board can solve a number of problems, especially where the misbehaviour is intermittent/inconsistent.
Here's an image showing the various 'brains' around your car. Clockwise from the top: ABS, Instruments, Central Convenience, Engine, Power Steering, Airbags and HVAC (heating +A/C).
In the middle, is the unit that arguably is in overall charge, referred to by a confusing array of different names, Onboard supply Control unit, Vehicle Voltage Control Unit, Body Control Module etc. It is the hub of all communication and control activity, co-ordinating everything else. It is located above the accelerator pedal on RHD cars.
What has been used to try to read engine fault codes? To the best of my knowledge, if the warning light has been on, there will be codes stored, they just may not be readable by every device capable of connecting to the OBDII socket. Try to find a local member or garage with VCDS, an aftermarket system that talks 'properly' to the VW-group systems; that should be able to find any and every stored fault code, in any of the above modules/ECUs.
If the door-related faults are happening every time, I'd say it might be a broken wire or two in the bellows, but if they are more unpredictable and sometimes work as expected, more likely the door-open microswitch on the lock module (or its wire solder joints onto the circuit board inside).