Updating this morning went straight to success page, with disabled maintenance mode. From there any and every admin page returned a header and favicon and pure white space on the completed page.
The blog was fine. The only thing to do was to disable the plugin through ftp, after which everything was normal.
The plugins page announced it was disabled, and announced a security plugin being disabled ( can't remember wording, but I believe it to be part of this plugin, rather than any other ).