Thank you for your response.
I have done that, but the PHP.exe window which opens up doesn't seem to do anything. It hangs around for a while and then closes - but no emails are forwarded. I can see that there are email in the inbox ready.
If I run "C:\wamp\bin\php\php5.4.3\php.exe" it opens up a blank PHP command window with "C:\wamp\bin\php\php5.4.3\php.exe" in the title.
If I run "c:\wamp\www\helpdesk\api\cron.php" it opens up a text file with the cron.php text in it.
If I run "C:\wamp\bin\php\php5.4.3\php.exe -f c:\wamp\www\helpdesk\api\cron.php" the php.exe command window opens up black, hangs for a few seconds and then closes again. Again, no email is picked up and added into OSTicket.
I'm really lost here...