Weak coffee I've just bought a silvia, and i followed all the instructions for startup, but i'm getting really weak coffee. What should i check/do?

The simple fact with any espresso making is that to get good coffee you MUST:

1. Grind your own freshly ground beans - grind matters alot. With a good grinder (a burr grinder, not a spice mill). This probably makes the most difference.
2. Pack the coffee properly using a solid heavy tamper (not the flimsy things you get with the machines)
3. Buy good coffee - shop coffee is old and stale. If you have to buy shop coffee get something like Illy - its much better than the others.
4. Ensure the machine is hot, and also that the portafilter is hot (leave it in there when not in use). Cold machine == sour coffee
5. Dose the coffee properly.
6. Adjust the above until the time taken to pull a shot is approx 23 seconds - you may need to 'sacrifice' some coffee to get to this point.
7. And in my opinion only use a double basket - single baskets are hard to use, don't pack well are weak, and inconsistent. A double basket uses more coffee but you get a way better result, more often.