So I just wasted another hour trying to work out why I couldn't select certain sketch contours for an extrude. Searching through the forums I found someone else with this problem, and it was caused by sketch visibility being turned off. As soon as I read this, I knew this was the problem I was having - I've been there a number of times before. The thing is when trying to select contours, it is completely unobvious that sketch visibility has to be turned on as I can ACTUALLY SEE the sketch. What does turning off sketch visibility even do? I just can't understand the logic supposedly involved here.
OK sorry, that was a bit of a rant, but is anyone able to explain how sketch visibility works?