For example:

[Fri Dec 14 10:01:36 2012] [warn] [client 90.202.217.210] mod_fcgid: stderr: PHP Notice: Undefined index: 507ad4da18944usr_ses_id in /var/www/vhosts/mysite/httpdocs/freichat/server/freichat.php on line 65, referer: http://www.mysite/
[Fri Dec 14 10:01:36 2012] [warn] [client 90.202.217.210] mod_fcgid: stderr: PHP Notice: Undefined index: 507ad4da18944usr_name in /var/www/vhosts/mysite/httpdocs/freichat/server/freichat.php on line 66, referer: http://www.mysite/
[Fri Dec 14 10:01:36 2012] [warn] [client 90.202.217.210] mod_fcgid: stderr: PHP Fatal error: Call to a member function fetchAll() on a non-object in /var/www/vhosts/mysite/httpdocs/freichat/server/freichat.php on line 812, referer: http://www.mysite/
[Fri Dec 14 10:01:36 2012] [warn] [client 90.202.217.210] mod_fcgid: stderr: PHP Notice: Undefined index: 507ad4da18944usr_ses_id in /var/www/vhosts/mysite/httpdocs/freichat/server/freichat.php on line 65, referer: http://www.mysite/
[Fri Dec 14 10:01:36 2012] [warn] [client 90.202.217.210] mod_fcgid: stderr: PHP Notice: Undefined index: 507ad4da18944usr_name in /var/www/vhosts/mysite/httpdocs/freichat/server/freichat.php on line 66, referer: http://www.mysite/

Is this something that can be fixed and if so where do I look please?

I'm using Freichat 7.5.3 on a Joomla 1.5.26 site.

try to install FreiChat 7.6 but keep the same module

OK 7.6 installed but still showing in Apache error log:

[Fri Dec 21 08:02:47 2012] [warn] [client 66.249.76.238] mod_fcgid: stderr: PHP Notice: Undefined index: 50d349c6cec58is_guest in /var/www/vhosts/mysite/httpdocs/freichat/server/drivers/base.php on line 134
[Fri Dec 21 08:02:47 2012] [warn] [client 66.249.76.238] mod_fcgid: stderr: PHP Notice: Undefined index: 50d349c6cec58usr_ses_id in /var/www/vhosts/mysite/httpdocs/freichat/server/drivers/base.php on line 136
[Fri Dec 21 08:02:47 2012] [warn] [client 66.249.76.238] mod_fcgid: stderr: PHP Fatal error: Call to a member function rowCount() on a non-object in /var/www/vhosts/mysite/httpdocs/freichat/server/drivers/base.php on line 138

Anything else I can do?

enable PHP debugging in the backend, load your website in the browser, after a while, check the freichat/freixlog.log file.
Post if you find any notices, inconsistencies or errors.

The friechat log appears to be not showing anything odd (as far as I know) ie:

January 21, 2013, 2:16 am: Inserted the user with the following data :: Username = Guest-9ul And ID = 1359063939 Default status = I am available

January 21, 2013, 2:21 am: main.php loaded

January 21, 2013, 2:21 am: Inserted the user with the following data :: Username = Guest-1n3 And ID = 1359073669 Default status = I am available

January 21, 2013, 2:22 am: Inserted the user with the following data :: Username = Guest-9ul And ID = 1359063939 Default status = I am available

January 21, 2013, 2:28 am: main.php loaded

January 21, 2013, 2:28 am: Inserted the user with the following data :: Username = neilmac And ID = 63 Default status = I am available

January 21, 2013, 2:40 am: main.php loaded

January 21, 2013, 2:40 am: Inserted the user with the following data :: Username = william And ID = 1307 Default status = I am available

January 21, 2013, 2:40 am: main.php loaded

January 21, 2013, 2:40 am: Inserted the user with the following data :: Username = william And ID = 1307 Default status = I am available

January 21, 2013, 2:40 am: main.php loaded

January 21, 2013, 2:41 am: main.php loaded

January 21, 2013, 2:43 am: main.php loaded

January 21, 2013, 2:43 am: Inserted the user with the following data :: Username = Guest-9w3 And ID = 1359206873 Default status = I am available

January 21, 2013, 2:48 am: Inserted the user with the following data :: Username = Guest-9w3 And ID = 1359206873 Default status = I am available


But I am still getting lots of PHP Warnings and Notices as follows:

[Mon Jan 21 10:00:16 2013] [warn] [client 31.110.108.176] mod_fcgid: stderr: PHP Notice: Undefined variable: freichat_theme in /var/www/vhosts/mysite/httpdocs/freichat/client/jquery/freichat_themes/basic/css.php on line 5, referer: http://www.mysite/component/comprofiler/userprofile/william.html
[Mon Jan 21 10:00:16 2013] [warn] [client 31.110.108.176] mod_fcgid: stderr: PHP Warning: include(/var/www/vhosts/mysite/httpdocs/freichat/client/jquery/freichat_themes//argument.php): failed to open stream: No such file or directory in /var/www/vhosts/mysite/httpdocs/freichat/client/jquery/freichat_themes/basic/css.php on line 5, referer: http://www.mysite/component/comprofiler/userprofile/william.html
[Mon Jan 21 10:00:16 2013] [warn] [client 31.110.108.176] mod_fcgid: stderr: PHP Warning: include(): Failed opening '/var/www/vhosts/mysite/httpdocs/freichat/client/jquery/freichat_themes//argument.php' for inclusion (include_path='.:') in /var/www/vhosts/mysite/httpdocs/freichat/client/jquery/freichat_themes/basic/css.php on line 5, referer: http://www.mysite/component/comprofiler/userprofile/william.html
[Mon Jan 21 10:00:16 2013] [warn] [client 31.110.108.176] mod_fcgid: stderr: PHP Notice: Undefined variable: chatroom_head in /var/www/vhosts/mysite/httpdocs/freichat/client/jquery/freichat_themes/basic/chatroom_css.php on line 32, referer: http://www.mysite/component/comprofiler/userprofile/william.html
[Mon Jan 21 10:00:16 2013] [warn] [client 31.110.108.176] mod_fcgid: stderr: PHP Notice: Undefined variable: chatroomimg in /var/www/vhosts/mysite/httpdocs/freichat/client/jquery/freichat_themes/basic/chatroom_css.php on line 39, referer: http://www.mysite/component/comprofiler/userprofile/william.html
[Mon Jan 21 10:00:16 2013] [warn] [client 31.110.108.176] mod_fcgid: stderr: PHP Notice: Undefined variable: chatroom_leftpanel in /var/www/vhosts/mysite/httpdocs/freichat/client/jquery/freichat_themes/basic/chatroom_css.php on line 44, referer: http://www.mysite/component/comprofiler/userprofile/william.html
[Mon Jan 21 10:00:16 2013] [warn] [client 31.110.108.176] mod_fcgid: stderr: PHP Notice: Undefined variable: chatroom_selected in /var/www/vhosts/mysite/httpdocs/freichat/client/jquery/freichat_themes/basic/chatroom_css.php on line 135, referer: http://www.mysite/component/comprofiler/userprofile/william.html
[Mon Jan 21 10:00:16 2013] [warn] [client 31.110.108.176] mod_fcgid: stderr: PHP Warning: Cannot modify header information - headers already sent by (output started at /var/www/vhosts/mysite/httpdocs/freichat/client/jquery/freichat_themes/basic/chatroom_css.php:310) in /var/www/vhosts/mysite/httpdocs/freichat/client/jquery/freichat_themes/basic/css.php on line 17, referer: http://www.mysite/component/comprofiler/userprofile/william.html

Any idea where I look for the problems? Predictably my hosting company say the errors are likely to be linked with the coding.

Can you try installing the latest version of FreiChat i.e v.8.0.1 , because it includes many significant changes and an important security fix .
Also the css files (the files that your logs are pointing to) do not include any PHP code (from v.8.x) so there is no chance of such warnings to appear in the latest version.

OK thank you, I have installed v.8.0.1 so wil check the logs again after a bit of use.

One thing though, with the new installation I can only see the chatbox on my homepage - is there a new setting somewhere to fix this?

There was no change in code that would cause such behavior . Have you assigned the Joomla module to be visible on all pages ?

Do you get any errors in the browser error console where the chatbox does not appear ?