Thanks for the very quick reply, Pau. I have no idea what the first part of your answer meant ;) but I found the setting allowing me to turn on captcha for anon requests. I already had captcha enabled but missed these settings when I went through the config files.

For info setting up captchas requires getting keys from the two captcha websites (http://www.google.com/recaptcha and http://textcaptcha.com/), adding the keys to config-private.pl then switching 'support_submit_anon' to 1 in config.pl.

Edit: and I've just noticed textcaptcha is missing from config-local in the github repo file so here's the code for it if one needs it:

# setup textcaptcha
%TEXTCAPTCHA = (
api_key => $DW::PRIVATE::TEXTCAPTCHA{api_key},
);