OK I got it working, thanks for all your help. The issue wasn't that I hadn't cleaned the switches out properly, but that I hadn't reseated the key caps properly after cleaning them.
It turns out that with the Das Keyboards you have to press the Key Cap down a little harder than I expected to make sure the key cap is fully reseated.
Looks like these things can take a bit more of a beating than I initially suspected, that's quite good seeing as my kids are little tornadoes of destruction.