Hi Nigel,
It’s great to have the display of errors turned on when in development (as of course you are) however notices like these – though ideally we wouldn’t be generating any – aren’t necessarily indicative of any deep problems as such.
In this particular case, because they are being displayed, they are stopping header information from being sent (as the notices are being printed before the code changing the headers is run) – but turning display_errors off would remedy this.
The process for this varies from host to host. Sometimes it can be done by simply changing wp-config.php so that the WP_DEBUG constant is set to false, or else you may need to do it from within your .htaccess or php.ini file – the best thing though is to get in touch with your host on this one.
I’ll definitely make a note for the dev team to investigate the generation of the notices here, however – thanks for reporting – and let me know if the above doesn’t help here.