Well, I've solved the GD issue by changing the extension_dir variable. Weird thing is that whilst php_info clearly shows mysqli installed and functioning, plogger install page still is looking for mysql. The folder that GD works from now has both mysql and mysqli dlls in place.
This paticular server only has the one site using mysql . All others use ms sql or oracle .
Oppps! my bad. It didn't occur to me that plogger was specifically looking for mysql, and doesn't need mysqli..... sorted now.