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I have set my location correctly in live.telldus.com - I have set an event, if front door opens (magnetic switch) and sun is down (0,0 offset), turn lounge lamp on. The sunset here is 21:21 tonight, yet the lamp turned on when I got home at 18:43 ... any ideas?

Perhaps you have occidentally programmed the wall switch with the house code of the magnetic switch, and thereby by-passing the Event?
Try erasing and re-programming the receiver, and then test the Event with for instance a "time of day condition".

I watch the event page and see the sun down condition passing, despite it being sunny outside with three hours daylight left.

If you open a device in the Scheduler, do you see the correct sunset time in the dialogue box?
Correct time zone setting?

Sunrise (04:49) Sunset (21:21)

TZ : Europe/London

So, all looks correct, yes.

Strange, I have used this feature myself and it works OK at my location.
You don't have anything else in your Event that could affect the behaviour?

If you run out of ideas, then file a support ticket at Telldus, http://www.telldus.se/support/index they are know to respond quickly.