The latest 1.9 Beta 4 update stalls at the Plugin Check page

I was doing a routine update using cvs of a working 1.9 Beta 4 site from 2007101506 (all updates todate have been fine) to 2007101508.05. The upgrade goes as far as the Plugin Check and then stalls. It does not display the table and does not allow any forward movement. It seems that the rendering of the page is incomplete. Tried changing to basic templates, but made no difference.
The site seems to be still functioning and can log in as standard user, but admin stuck in a loop.
Can provide login if required.