Well if'n I ain't the dumbest thing since the day Professor Dumb of Dumb University purchased an electrified Dumbing machine...
Only today did it occur to me to actually view the log of the install. Duh. And it said (in part...)
\tInstalling with target /
\t/System/Library/PrivateFrameworks/Installation.framework/Resources/pax:
\tcould not read compressed data
\t:
\tUndefined error: 0
\tERROR:Could not read archive file. - pax: End of archive volume 1 reached -
\tERROR:Could not write file. - pax: WARNING! These patterns were not matched: -
\t./Applications/Mail.app/Contents/Info.plist
\t./Applications/Mail.app/Contents/version.plist
\t./System/Library/CoreServices/SecurityAgent.app
...(and much much more)
So at least that gives me something to go on with. Have only just discovered this so not sure what I'll be doing just yet.
And sure enough, in /library/receipts the 10.1.1 package was much smaller than it should have been. Will play around and post an update.