I don't know if this will solve your whole problem but one thing is staring me in the face:
$data = mysql_query("SELECT title, first_name, surname, addr_line_1, addr_line_2, city, postcode, tel_number, mob_number, email FROM members where id = '$_post[id]'")or die(mysql_error());
$result = mysql_query($data)or die ("Query failed: " . mysql_error());
There should be no mysql_query() on the first line. You can also delete the or die at the end of that line. Instead, try this:
$data = "SELECT title, first_name, surname, addr_line_1, addr_line_2, city, postcode, tel_number, mob_number, email FROM members WHERE id = '$_post[id]'";
$result = mysql_query($data)or die ("Query failed: " . mysql_error());