What time is best to get there?
I think we normally arrive between 19:30 and 20:00, after watching "Dinner for One". We walk from Hauptbahnhof - just follow the crowds. There will be a queue at security. 18:00 IMO is too early, but they will shut the entry points when they decide it's full. I think this normally happens between 21:00 and 22:00.
I know it is a free event, do we need tickets for entry into the secure area and are they available on the day?
No (and therefore no).
Can we bring our own drinks to keep warm?
Yes. You can take in plastic bottles up to 500 ml. No glass bottles. No cans. We usually take a couple of Coke bottles filled with vodka. Don't try to conceal them when you are searched on the way in.
What time does it open and how early do we need to start queuing?
I'm not sure. It's open in the afternoon, I think. It's more of a melee than a queue when it's busy, but it doesn't take ages. Get there when it suits you.
Are there any seating areas and toilets in the secure area?
No. Yes - very well managed, 50 cents a go and well worth it.
As of December 2015, you cannot take most bags and backpacks in. The site is open on the afternoon and evening of the 30th for rehearsals etc. so if you are in the area, go along and ask security what you can and can't take in. They are professional and helpful.
Have a great time. We're going for our 7th year running.