Hi Angus - can I assume you're using Chrome? Unfortunately they've stopped the necessary support for this to work, as stated here -
https://support.google.com/chrome/answer/6213033
I use Firefox which allows me to make the necessary exception & to access the puzzle - perhaps you could give that a try? Just in case you're unaware, you can have as many browsers on your PC as you like, so that wouldn't be a problem.
https://www.mozilla.org/en-GB/firefox/new/
I hope that's been of some help, if not you could try starting a thread in Technology to see if anyone can offer more knowledgeable assistance :-)