First off, thanks AlanH for taking the time to respond.
A bit of searching on YouTube produced the answer. It turns out to be a bit of a newbie blunder I suppose, but an easy mistake to make.
Basically I had linked my Spotify Premium account to the Echo Spot and I thought that was it, but it isn't. The other step is to select Spotify as your default music service. I did that from the Alexa Android app (although you can do it from a PC exactly the same).
Go to settings>Music & Media (under Alexa Preferences)>Under "Account Settings" there's a box for "Choose Default music services. This is set to Amazon Music by default (obviously), so just select "Spotify" and click Done and you are, er, done.
I don't know what the "Default Station Service" button does on the same page however, it seems stuck to Amazon Music so I'll leave that.
I just need to get my Synology NAS Audiostation linked now and I am ready to Rock & Roll!