Go to shell and pick up some Optimax in one of those small red containers. Then run your car almost dry then detatch the fuel tank. Empty out all the crapy cheap petrol and wash it out with a bit of water or something. Just plain water, I wouldnt use any cleaning stuff. I wouldnt have thought water would damage your tank in anyway. Pop the tank back on. Then put the optimax in, then pop down to shell again and put more optimax in. Then go for a blast. Go WOT and it should help force any crap along. Optimax has cleansers in it and will help clean your injectors etc...or so they say. I must say though that I recently started using Optimax (used to use Sainsburys 95 ron) and I must say it runs far better now. No more power, but it feels smoother and my flat spot seems to have diminished slightly, but that might be down to the weather.
Taking the tank out might be a task. There is usually a drainage plug at the bottom, or something similar. Just use that, and flush it out with a hose pipe. Me and my dad did something similar after he put petrol in my mums 1.8 diesal fiesta. But there wasnt a drainage plug there, cant remember what it was, cos I didnt actually take it out. I think it was a pipe, so it might have been the fuel line.
Paul