Having just moved my forum and solved a few problems with some help I now find one with the extensions - the 'not checked' ones are a darker yellow that the others and when I try to enable them the check box changes to the tick mark (but there is none of the usual whirling green logo thing) and when I move away from that particular screen and return they are no longer checked? Nothing much in the docs about this...

EDIT: Oops no I have it - there entries in conf/settings.php were missing!