You could try railbookers.com
We've just booked from Macclesfield to Cologne via Brussels with them & it works out a lot cheaper than booking independantly.
We were told that if you book using the Thalys train they can quote you a fare from London - Cologne. If you try to book via Eurostar or the ICE you have to book each train seperately which will work out more expensive.
Have you also tried the Deutsche Bahn website?