Thanks for pointing out that oscity.eu is online again. After some searching I found the data source.

Unless I am very much mistaken, here you can find the files. These are zipped TIFF-files. They look like grayscale images that can be convertedeto height data/maps.
They are pretty much completely white, but remember: 1 meter is a hill, and 10 meters is a mountain in the Netherlands.

In the coming days, I will try and make some sense of this.
To be continued...

H