Permissions problem on forum posts

I am trying to set up a forum on my site. I want anonymous users to be able to post.
Whenever an anonymous user posts to the forum, they get http 500 unless I enable all the permission
"administer comments enabled"

however I dont want to give anonymous users this permission. Authenticated users can post no problems. Any ideas ?

Modules used:
Comment
Forum
Advanced Forum
Akismet

http headers inspected duping post:--

http://www.chickens-at-home.co.uk/comment/reply/190

POST /comment/reply/190 HTTP/1.1
Host: www.chickens-at-home.co.uk
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.0; en-GB; rv:1.9.2.3) Gecko/20100401 Firefox/3.6.3 (.NET CLR 3.5.30729)
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-gb,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive: 115
Connection: keep-alive
Referer: http://www.chickens-at-home.co.uk/content/hen-house-matters
Cookie: __utma=220998858.284864954.1261674142.1272371480.1272376483.19; __utmz=220998858.1270842679.13.10.utmcsr=google|utmccn=(organic)|utmcmd=organic|utmctr=chickens%20at%20home; SESS2805e1076c2a87da661dcb515154c0dc=84b4d2bff991020894a3d8f589125b78; textsize=100; __utmb=220998858.4.10.1272376483; __qca=P0-516980729-1272374672702; has_js=1; __utmc=220998858; comment_info_name=Anonymous
Content-Type: application/x-www-form-urlencoded
Content-Length: 146
name=Anonymous&mail=&homepage=&comment=testpost&format=2&form_build_id=form-9d2a3f6f5cc02a8e1a2e38658cffda48&form_id=comment_form&_op=Save&op=Save
HTTP/1.1 500 Internal Server Error
Date: Tue, 27 Apr 2010 13:57:27 GMT
Server: Apache
X-Powered-By: PHP/5.3.2
Expires: Sun, 19 Nov 1978 05:00:00 GMT
Last-Modified: Tue, 27 Apr 2010 13:57:27 GMT
Cache-Control: store, no-cache, must-revalidate, post-check=0, pre-check=0
Set-Cookie: comment_info_name=Anonymous; expires=Wed, 27-Apr-2011 13:57:27 GMT; path=/
Set-Cookie: comment_info_homepage=deleted; expires=Mon, 27-Apr-2009 13:57:26 GMT; path=/
Set-Cookie: comment_info_mail=deleted; expires=Mon, 27-Apr-2009 13:57:26 GMT; path=/
Connection: close
Transfer-Encoding: chunked
Content-Type: text/html; charset=utf-8
----------------------------------------------------------

Thanks !!