It is you with the tank that seems empty but will only take 22 litres, isn't it?
Drain the tank. Put the petrol in a bucket overnight. Put something over the bucket so that rain doesn't get in. Next day, slowly decant the first half into another container. Get rid of the rest.
In the meantime, buy a new fuel filter (looks like a dirty coke can, under the car), fit it (make sure it's the right way round). Pour the good petrol back.
This may or may not get rid of the rough idle, but it's a good place to start!