Thanks you @speechles I thought that might be the case.
Fortunately I have found a partial workaround for this issue by adding a standard range: bytes= header with the .Seek(). The only thing that is now not clear is if I can avoid reopening the content and using Pause() , Seek() and Resume() to forward a new header range header. It is doing some seeking just not what I expect. I am still hoping that this will be possible.
I believe I may have seen you on the Emby forum and thanks again for your help. Direct play for Emby would be useful too.
Martin