You can fix this by editing your site.php file.

This file is found in your root/config/site.php

add these two lines of code:
define('BASE_URL', 'http://www.yoursite.com');
define('DIR_REL', '');

where obviouslyhttp://www.yoursite.com is actually your website's URL.
Your site.php might already have a these two lines - so rewrite the 'DIR_REL' so it's empty.

yes the site.php already had the lines in and i removed the /concrete so that the DIR_REL was blank but unfortunately gave an error message as page not found.

in the .htaccess file i have the following and I think the problem s that im using a subfolder for the site.

content of .htaccess >

# Copy and paste the following code into the .htaccess file
# in the public_html folder of your hosting account
# make the changes to the file according to the instructions.

# Do not change this line -
RewriteEngine on

# Change yourdomain.com to be your main domain.
RewriteCond %{HTTP_HOST} ^(www.)?portchesteronline.co.uk$

# Change 'subfolder' to be the folder you will use for your main domain.
RewriteCond %{REQUEST_URI} !^/concrete/

# Don't change this line.
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d

# Change 'subfolder' to be the folder you will use for your main domain.
RewriteRule ^(.*)$ /concrete/$1

# Change yourdomain.com to be your main domain again.
# Change 'subfolder' to be the folder you will use for your main domain
# followed by / then the main file for your site, index.php, index.html, etc.
RewriteCond %{HTTP_HOST} ^(www.)?portchesteronline.co.uk$
RewriteRule ^(/)?$ concrete/index.php [L]