Make sure the captcha plugin is enabled.

and you have the GD library extension too :)

Also, true type font is installed on the server.

I had the captcha plugin installed. i've moved to the bottom to see if that helps.

Where do I find the GD library?

hmathias, if it's not a GD library, it's possible you have BOM encoding problem. Check wether your user icons are displayed correctly on the site. If they're not displayed, you have to find a file which is written in UTF-8 with BOM format and convert it to UTF-8 without BOM.

I've checked the GD library and the true type font and they're fine. And the icons are displaying correctly. Any other ideas?

http://moodle01.st-davids-coll.ac.uk/elgg/account/register.php

it used to work when we were on Windows but we recently moved to Linux and this problem started

Inspecting your page shows that a captcha token is being generated:

<input type="hidden" name="captcha_token" value="19298d2409b3e71300f8bb7c3eaa90a8">

I notice you have a 'Accept Terms and Conditions' plugin of some kind. Did these two work together on your previous Windows server?

I'm not sure. I'll try disabling this and see if that helps

Wow that seemed to do it!! Thanks for that.

I would still like our students to accept some terms and conditions in using the site. Is there a way of doing this?