Page/Post Preview Not Working

I have been developing a wordpress version for my choir. This was developed in https://www.birminghamchoralunion.org.uk/wordpress/?(which NOW for some reason shows the ‘old’ site) while the original site was live in https://www.birminghamchoralunion.org.uk/?(which now shows the ‘new’ site).
While this was the situation, I could Preview all pages and posts successfully.

I put the new version live on Monday so that the wordpress version is now in the root directory. ?SInce then I cannot Preview any draft page or post. ?I have tried following the these instructions but it makes no difference

https://example.wordpress.com/?p=1234

this becomes

https://example.wordpress.com/index.php?p=1234

then the preview works

now i had to edit one line in a WordPress File for this if you want to have the above change done automatically edit the file below im using WordPress 3.0.1

/wordpress/wp-includes/link-template.php
(wordpress is the install directory) (incase your using the root then just locate the wp-includes folder)

line number 161
it reads as:

$permalink = home_url('?p=' . $post->ID);
copy this there:

$permalink = home_url('index.php?p=' . $post->ID);

& save the file back

I would be very grateful for any help as I now have “non-techs” adding posts!!