The petrol tank is fairly simple. I haven't removed a Stilo tank but a few others and they're pretty much the same.
You need to lift up the rear seat (usually... otherwise it'll be the boot carpet) and find the hatch for the fuel pump.
Remove the hatch (3-4 M6 bolts), disconnect the electrical leads to the fuel pump and then remove the metal collar that holds the nylon fuel line terminals pressed into the top of the pump. They'll just pop out very easily (O-rings on nylon stubs). Don't twist them or force them.. they'll be brittle and could snap and I guarantee that a new one (with the glued on plastic fuel line attached) will be tricky to find.
Leave the pump in the tank but put a plastic bag over the fuel line terminals and in the fuel pump receiver holes, to keep the dust out.
Drain the tank as much as you can using a little pump or a syphon hose. It doesn't have to be *completely* empty but you'll reduce the risk of spillage later, if the tank is empty.
The tank is connected to the filler pipe by a clip (Jubilee or a Klic type). Undo that, get it well out of the way and then pull the filler pipe (which is flexible/concertina'd) off the stub on the tank. Did I mention have some container handy to catch any petrol that comes out... and best not to be smoking when you do this part.
Once you've mopped up all the spilt fuel, stuff a huge rag, or put a plastic bag over the end of the fuel tank stub held on with tape, since an elastic band will snap as soon as you get fuel anywhere near it.
Unbolt the tank. It'll be 3 or 4 M8 bolts or nuts. The tank should then be free of the car and can be eased off.
The only complications I can foresee are that the Stilo has a torsion beam and I dunno if that's going to interfere with removing the tank. Hopefully the tank will clear the beam, even if you have to take it out at an angle.
The exhaust might also get in the way but on the JTD the pipe passes next to the tank rather than underneath it, so your petrol might be the same. If you have to remove anything then it'll be the back-box, which is likely to be rusted to the rest of the system... so hopefully it's not in the way.
Ralf S.