A Possible fix for the "&" problem.
I Am NO Programmer an the fix may not work for other people so
BACKUP your file, but it does work for me.
in /include/class.xcart_seo.php look for ..
Code:
$query = preg_replace(array('@&@i','@&@'),array('&','&'),$query);
Replace with ..
Code:
$query = preg_replace(array('@&@i','@&@'),array('&','&'),$query);
all i've done is remove the amp; from the line of code & my checkout works a treat, anyways hope its useful to someone else.