Thanks William! Thanks Andy!
It is very easy to look to a Google update whenever something happens with traffic. On this occasion the Page Layout update times with a drop in ranking for these keywords - I've slipped off the bottom of page 1. Thanks for reminding me to look closer to home in the first instance - indeed it looks like the drop in rank is the more likely cause of the drop in traffic.
Andy - thanks for the point about the login link as well. Alas I inherited the site when I bought the company and it is not something that the makers of the platform want to change. I will drop them another line about it though.
Thanks for helping!
Cheers,
Simon