I had to remove the fuel tank from my car, but for different reasons. The Bentley manual as a helpful guide, but it's still an awful job, it took three guys to do it. Normally a fuel tank would be steam cleaned to remove contaminants, but obviously that can't be done while it's still attached to the car. My advice would be to see what you can do through the access panels. You might be able to siphon and sop up whatever's in there first, then spray or pour in a cleaner and work it around, siphon that out, repeat the process. Once everything's back together, change the fuel filter each month for the first two or three months afterwards.