I have a 2012 CAAD 8 and I think it has a 100mm stem currently.
I’m keen to try a longer stem to gain a lower position on the bike but struggling to;
a) figure which length and angle to go for and
b) figure which size stem I’d actually need (clamp sizes ect)

Is anybody able to confirm that the C4 Cannondale stem I currently have is indeed 100mm and also what size clamps I’d need.
Any other wisdom on how to decide would be great too, I’ve been looking at the heavily discounted ones on Planet X and Wiggle, not looking for anything fancy and not overly concerned by weight.

Different size bikes have different length stems, so no one will know your stem length without a bit more detail. Dimension wise it’ll be a standard stem, 31.8mm clamp.

Just going lower will make things feel longer. Try a 100mm with a greater drop on it.

This calc shows you where any combination of spacers, stem angle, and stem length puts the bars compared to your current set up.

http://yojimg.net/bike/web_tools/stem.php