Can't seem to drop into a roll while running. Can rise into a run from a roll but not the other way round. Without this ability some of the puzzles are impossible, any help?

Did you try pressing ... X, I think, while running? While you can normally use down to crouch, it doesn't work for dropping into a roll.

Alternatively, it's possible that your keyboard isn't registering keypresses beyond a certain number. Make sure you are only pressing Shift, the direction key, and X.

Thanks Qu4Z!!! That's the trick! Shift + Left Arrow or Right Arrow then while running hold X to roll. I was stuck for ages at the Scrab Nests because I couldn't roll quickly enough.

No problem. Glad you got it working.