I know what's involved in dropping the tank as i have a VF and why i said it's a PITA.... but it's the best solution...
As that's not possible for you, then surely a thin arm can reach somewhat into the tank through the fuel pump opening and push a long hose into the other half of the tank so you can suck out the remaining fuel. Alternatively, you can cut yet another hole in the vehicle floor so you can take out the fuel sender. Then you can pump out any bad fuel from that side of the tank. Removing both sender and pump will allow a better cleaning and inspection of the tank while its still installed within the vehicle.
Commonly, fuel contamination is caused by water and/or particulates as a result of poor fuel handling and/or pump maintenance at the pertol station itself. Another type of fuel contamination is what's known as phase separation which occurs in fuels containin alcohol (E10, E85 blends). Phase separation occurs over time as the fuel sits within the tank (since it continues to absorb water from the air). Less common contaminations are from some chemical inadvertently added to the fuel at the refinery (in one such case, when cleaning agents weren't corectly flushed from refinery equipments, this resulted in a real bad batch of fuel causing millions in damages).
However, without knowing what form of contamination you have suffered, you don't know what damage has occured. Some chemicals may soften the plastics your tank and/or hoses are made of, damage injectors, damage O2 sensors or any other components within the engine that fuel or combustion gasses touch. In such cases your only option could be component replacement.
So just to be sure, it would mean you'd need a sample to send off for analysis which can be expensive in itself. As a minimum, you should pump at least two litres into a large clen and dry coke bottle and let it sit for a day with cap on. Then you can inspect the fluid and see if there are particles (can check if metalic or biological using a magnet) or if there is any stratification of fuel (indicating water? or some other chemical?).
If the sample bottle of fuel looks clean as, your problem could simply be years of using 91 which has formed varnish (which occurs on both metal and plastic tanks and lines), and then using some e10 which will loosen any varnish which makes its way to your injectors and blocks them up. Or if your unlucky it could be some other chemical contamination and much more of a pain to deal with (but i'd think that's be on the news)...
So, once youve flushed the tank, i'd add 10 ltrs of 98 and a bottle of injector cleaner and then disconnect the fuel line from fuel rail and add an extension hose before putting the car into run to flush the line clean for a few seconds. Then i'd pull your injectors and see if they are clean or whether they are rusted looking with bubbly deposits around the injector outlet ports. In any case i'd clean them as below:
Then i'd install injectors and refit fuel line and start the car to see how it idles. If ok then i'd fill up with 10 ltrs of 98 and another bottle of injector cleaner, then go for a 50km drive. If all is ok with no DTC's, i'd fill up with 98 and a bottle of injector cleaner and drive as usuall. When tank is near empty, id replace engine oil and filter before filling up (injector cleaner can shorten engine oil life).
