I'm also in Melbourne and trying to find a solution to this... Must be the type of riding we do!
Yesterday I identified the issue with the broken cable spool. What started happening to me was that the cable jumped out of the spool and my shifters went past the limit on the spool - the end result is the same that you can't shift up or down, and your rear derailleur is stuck in the hardest gear.
So now that I've identified it, I'm keen to get the Ratio kit. I ONLY need the small, red plastic part (which is black from Ratio- about $10) . But I wanted to know - has anyone taken the left side shifter cable spool and donated that to the right side? From the Ratio site it doesn't look like you need to specify left or right side, but I'm assuming it might just be always right-side.
I'm not keen on a hack job for this like some people have done - just feels like more effort than it's worth - so would like to do the 1-for-1 replacement.