At 20k the saving is not going to be massive and there is more to consider than MPG, what works for you will depend on whether you actually like driving or not.
If you like driving that being light on the pedal for the commute to have a decent car for other times might be worth it, I have switched back and forth between petrol and diesel, diesel have generally done 50-60mpg, the petrols have done 35-45, doing a commute Bristol to Maidenhead, I found very little differences in reality, the diesel would eat tyres and brakes due to weight and be no fun on other driving due to weight, which negates some of its saving IMO.
I think it would be a wash really over what was cheaper, what is better will come down to your driving preference. Also what will the cost to change versus what you have been, if you don’t need a new car that has to be considered.