Hi Jason Spinney, and welcome to the forum
If I run it as a dual system for a while until familiar with Ubuntu, will I then be able to ask Ubuntu to take over from Windows and replace it?
It shouldn't be too difficult to use gparted to remove the Windows partition, then resize the Ubuntu one(s).
I use a home based WD mycloud for back ups - will Ubuntu be able to back up to this and will I be able to access files from it?
I've no personal experience with the WD Mycloud .. but I own a Buffalo Linkstation NAS and have no problems with that, so I'd presume you'd be OK.
Does itunes work with Ubuntu?
NO .. some people will tell you there are applications that WILL sync with iDevices .. don't listen to them .. Apple keep changing the database format on iDevices, and every time they do they break Linux compatibility so the next time you sync you screw up the database resulting in an iDevice that can no longer have stuff added to it (even in Windows/OS X) without being formatted first.
Apple products just don't work well with Linux .. not a Linux problem, more that Apple keep breaking the compatibility.
Stick with Windows for iTunes .. be that a dual boot settup, or running Windows/iTunes in a virtual machine in Linux.
Will I need anti malware software such as Hitman pro or Malware bytes? Can I ditch Bitdefender?
As long as you're behind a NAT router you will NOT require AV or anti-malware in Linux.
Will cloud based storage such as Google drive and Dropbox work with Ubuntu?
Will Sonos controller work with Ubuntu?
According to this:http://forums.sonos.com/showthread.php?t=28539
someone managed to get it working in WINE .. but I'm more inclined to say NO or WITH GREAT DIFFICULTY
Again probably best to dual boot .. or run Windows in a VM
Does Spotify work with Ubuntu?
They're planning on it .. and currently have a working Linux preview available:https://www.spotify.com/uk/download/previews/
Will photoshop work with Ubuntu?
I've seen Photoshop CS5 Portable running in WINE .. but again I'd say WITH GREAT DIFFICULTY
But there is always the GIMP which is a free alternative to Photoshop .. there's a learning curve involved, but it's VERY capable.http://www.gimp.org/