i guess it varies from person to person. We didn't get there till about 10 to 10, went straight through, and were only waiting a very short while till we were let on to the platform.
You have to check in 30 minutes before. When we came back from Disneyland Paris in 2007 we didnt realise we have to check in 30 minutes before and they wouldnt let us through so we had to change our tickets and get the last eurostar train from Paris it was a nightmare!!
We got there about 8.30am in September which was far too early - there's not much to do and Lucas got bored after a while. Next time I'll probably leave it until 9.30am at the earliest.
Having just done this and living 10 mins from the station, we arrived just after nine and waited inside for the rest of our party. We went upstairs and checked in around 9.30, but it's so rushed having to put your bags through, go through the detector and then passport control if there are lots of people behind you! If you have a buggy then I'd suggest going up at around 9am as if you are like us, we pile everything on it then realise we have to take it all off to be scanned! tkae things for little ones and you'll be fine.
One point I'd like to make though, is they ask coaches 1-4 and 15-18 to go through first down to the actual platform after checkin to avoid huge crowds, but the lift is tiny! The platfrom is poorly covered and the weather we had to wait in was horrid! We froze!!