Having now flown back from Schiphol, I can confirm the arrangements there for anyone wanting to bring their bike back in a box.
First, it's so easy to cycle right up to the terminal on the dedicated cycle paths. Then you need to look for the lift from the arrivals level down to baggage and storage, just past the information point. That's where you buy the boxes, but note there is no sign mentioning cycles, and there aren't that many people around to ask.
The boxes are very sturdy, and large enough to take a road bike with both wheels still on. Important note - there are no packaging materials available, so if you want the peace of mind of having a well-padded bicycle, you have to acquire this beforehand. I actually didn't do this as the box looked strong enough, but I did take some duct tape to strengthen the box joins. The box is 22cm wide, so it's unlikely you will get away with just turning the handlebars - I had to take them off and attach them to the frame with the tape.
Be prepared from some jiggery pokery to get the box to check in. It's 1.75m wide and therefore will not go into the lift without taking it off the trolley. Also note that, at the oversize baggage area, they lay the box flat. so it's worth padding the derailleur to protect it.
Happily, the bike arrived at Heathrow undamaged. Finally, as many others have mentioned, don't forget to let your tyres down. If you're travelling with BA, they remind you at check in.