Gez,
Sorry for the long delay in getting back to this. I missed the post in our forums. The Sendy extension is nice, but not a lot of support requests for it...
As to your issue, that's a new one for us. We're on a slightly older build of Sendy, but I'm seeing that this site is running into an issue with signups in our module at the bottom of our page too.
Let me investigate the issue on our end to see if it's related.
While we do that, can you let us know if you're still having the problem. If so, let us know:
* Your Joomla version number
* Your Sendy version number
* Anything else you think may be relevant.
Thanks!
Alex