Dreamweaver does not support the MySQLi connection, (or the pdo one). Check your php config has the standard MySQL connection script installed and use this for now, (don't know but maybe CS5 will support MySQLi or pdo ).
You're correct! I just managed to get my connection to work. I looked in my php.ini file and it only had extension=php_mysqli.dll at the bottom of the page (the dll file to which this refers was in the ext folder). So, I just added extension=php_mysql.dll to the end of the php.ini file then downloaded the PHP 5.2.13 zip package from http://www.php.net/downloads.php and then copied the file php_mysql.dll into the ext folder. I got the clue about mysqli not being supported after i'd carried out a Site > Advanced > Remove Connection scripts - that at least gave me a message "Your PHP Server doesn't have the MySQL module loaded or you cant use the mysql(p) connect functions" - before i did this i just got a useless "An unidentified error has occurred" which left me no further forward. Thanks for your help!
Oh and I probably am stupid to answer my own question but the doc
umentation / information available when dealing with issues such as these is woeful (imo) - thank goodness
we have many good people on this board to help out