So, after playing one game last night with no problems, tried to start another a few hours later and the client would not load.
After a few minutes of messing around with the Java cache etc, I got the game cancelled - apologies to my opponent - and tried getting the latest Java update. Which failed :(
Uninstalled the current version completely, no Java showing in control panel, deleted any and all Java folders... and both the offline and online 6u24 installers still failed with 'the wizard was interrupted'. Not helpful.
Much googling later, I finally discovered that downloading from a different source, not java.com but rather Oracle (http://www.oracle.com/technetwork/java/javase/downloads/index.html) gets you a different, slightly larger installer. Which worked :)
Heaven knows Java is a great idea. But I really do wish that there was a windows version produced by somebody a bit less useless than Sun/Oracle...

Changed the title, Woodstock