If I enter timeline:/ in the dolphin address bar I
get a list of folder icons: today, yesterday etc

Unfortunately, all these folders are completely empty.
(Yes, I have opened the odd file in the last few
days and today as well.)

Is there a trick to get this working (or does it only
work as well as the other nepomuk functions I've tried
unsuccessfully to use)?

By the way, the strigi function is turned off for me - because
it was taking multi-gb of memory and 2 of my cores
maxed out! Do I need strigi for timeline?

thanks in advance,

Strigi is required for the timeline I believe as it needs to know about your files.

Please ensure no unneeded directories are enabled in System Settings > Advanced > Desktop Search

Timeline still is not working as expected here (KDE SC 4.5.2): https://bugs.kde.org/show_bug.cgi?id=255821

Strigi is enabled but does not do anything at the moment. I have downloaded some files today in the last 12 hours but "today" remains empty.

I can confirm this issue on KDE Trunk. Dolphin search shows a number of directories altered today, but nothing is shown by timeline:/