Well, this might not be of any help to you Lachlan, but I thought Id reply anyway! ;)
We just recently experienced a sour batch of coffee at work. At first it didnt seem obvious until later in the day when the boss lady and I tried the coffee and nearly spat it out on the floor!
A regular who comes in for his shot said it tasted bitter, especially after I had done all my usual steps to produce the perfect shot for him and I was left scratching my head in confusion. As the day went on, so it seemed the coffee soured even more! Probably because it was warming up inside the shop and the beans were letting off more of their odor.
We removed the offending beans from the hopper and decided that their smell was odd and that the coffee flavour was coming from them. In the end, we just chucked them out. Not much more we thought we could do. Especially as our supplier is not just around the corner! (rather a full days drive away!)