Williams are mk1 wheels so for Matt to have them on his car, I'd guess they're the same bore size. Might need to check this out on a car wheel info site etc. Or might be in the guides section actually.
I'll check the standard 172 wheel offset tomo, I've just taken mine off. Am sure they said et48, but I may have misread it so it might be et43 as said above. et37 they will stick out more, et is something like the center of the wheel in comparison to the hub of the wheel, so a positive number means the centre is sticking out from the hub. Bit hard to explain without pictures, plenty of info about it on the net though. If the Ultras are 7" which I'm assuming they are, an et37 means they will stick out 6mm more than standard wheels if standard wheels are et43. Which will give you a 12mm wider track.
PCD is about the bolt spacing, 4x100 for both mk1 and mk2 IIRC. The bore is the size of the centre hole. So alloys are available in different sizes to accomodate for different cars. If its too big for your car, then you can get spigot rings which fit in the middle of the hole to decrease the size to the correct diameter for the car. Very cheap to get. This very important as this is what the wheel sits on. If the wheel bore is bigger than the car's then the wheel will basically be resting on the wheel nuts, which is dangereous as that's not what they are designed to do, they're just there to keep the wheel from falling off.
