Two Column issues – width settings?

On my blog I seem to be currently having some issues with my columns. I’m using a two column theme but this issue is still occurring with all other themes.

The homepage is fine (http://www.thealice.co.uk/) but when you go into any posts the left sidebar drops below the middle column (http://www.thealice.co.uk/2010/11/3-months-of-baby/). This also happens with pages as well (http://www.thealice.co.uk/about/).

I’m pretty sure this has happened to me before and it’s something to do with a width setting somewhere… but I can’t for the life of me remember how to fix it!

Any help much appreciated.