It will no invalidate the warranty!!!
Disconnecting the oil level sensor is a bit tricky on the EU20i, you can do it on the EU10i with relative ease.
This is a oil issue, the oil goes a bit thick once cold, if you use the correct grade of oil all will be well, failing that keep the generator somewhere warm then it will start and continue to run.
Recomended oil for the EU20i is SAE 10W - 30 or SAE 30, service classification SE, SF or SG
How can they charge £500 for an ignition control unit??? the unit in question is roughly £70, fitting takes less than 15 minutes! are you boaters being ripped off?