I must have forgotten that I changed the default message when I first set up the form (introducing that typo in the process), which explains why I couldn't find the exact text I was seeing anywhere in the plugin code. While I was searching for how to do this, I forgot to look "below the fold" and I also didn't think of creating another form to confirm the problem, which would probably have pointed me in the right direction. The perils of working late at night...
Thanks for your quick reply,
Shi.