Have now resolved this with the hosting company - they were unable do much about the open_baseurl function as we are on a shared sever.
They amended the PasswordHash.php file in a similar way to a link here somewhere and registration is all OK now:-
Added the below code at line 51
======
if (@is_readable('/dev/urandom') &&
======
instead of
======
if (is_readable('/dev/urandom') &&
======
More info here:-
github.com/jenssegers/CodeIgniter-Phpass-Library/issues/1
Jim