Not sure about this but I think Thalys ditched Duesseldorf and now the final stop for all trains is Cologne. However you can hop on an ICE or EC/IC to Ddorf from Cologne HBF and pay by credit card on the train. That way any possible Thalys delays coming in from Brussels don't hamper with reserved seats, you'll always find room in 1st for Cologne - Ddorf.

So book London ( Waterloo ) - Cologne on the Eurostar site and just take the next suitable train to Ddorf once you arrive at Cologne.