visionary:If I plug my (networked via homeplug) laptop in to it for video and use the SPDIF on the laptop direct into the "Digital in" on my Denon I will have internet and also media streaming from my desktop PC?
Yes.
Is there any benefit in buying any other solution in terms of better sound or video or facilities? I was thinking of, e.g., Apple TV but I'm starting to wonder if there's any point.
For a long time I avoided the Apple TV because of all the negative press. Then I realized that the journalists actually didn't knew what they wrote about. Almost any limitations in the ATV might be bypassed with additional software (free or inexpensive).
The most obvious improvement, compared with your solution, is of course that you'll get rid of the cables.
I can see that using a NAS would mean not needing my desktop to be on all the time but that seems about all.
I've got a NAS, but I've found it more convenient to stream from my laptop or syncronize whatever material I want to the ATV hard drive(s).
To use a NAS, your streaming device (ATV or whatever) needs to communicate with the NAS, which has to run supporting software. In short, this will usually result in compatibility issues. If you chose the wrong NAS for the wrong streaming device, you might end up with a very limited choice of file formats.