I have upgraded the PHP to 5.3.3 and the configuration now seems to be working, no more white screen! Thank you for all your help.

This is the last line from the error log on the server after trying to access the configuration in Joomla, don't know if this helps at all.

[Tue Feb 14 17:16:51 2012] [error] [client 86.165.157.157] PHP Fatal error: Call to undefined function json_encode() in /var/www/vhosts/blueoceanmarine.co.uk/httpdocs/administrator/components/com_sef/views/config/tmpl/default.php on line 56, referer: www.blueoceanmarine.co.uk/administrator/...amp;task=showUpgrade

I've installed the paid version of JoomSEF 3 and the Virtuemart Ext bundle successfully using Joomla 1.5.23 but when I click on the configuration tab for JoomSEF I get a white screen and the message 'ARTIO JoomSEF Configuration file Writeable' but nothing happens and I cannot set up the configuration. My hosting is on an Apache Web Server with PHP Version 5.1.6 , I've searched the forums and found a couple of people with the same problem but no solution, any help appreciated.