I’ve been working on a clients song for a few days and today the whole mix seems to only come out of the left speaker.
» I have opened other projects and all working fine (so not a speaker issue)
» I’ve checked each channel (drums, guitar etc) and nothing suspicious there
Any ideas guys?
Do you see output in both channels in the meters?
VST connections/control room properly set?
In the Mixer, is signal showing on both channels in the output channel?
Do the Output Bus settings match those in the working projects?
mono is better 
Does playing a pre-recorded mp3 outside of Cubase sound fine?
Yes.
It’s not even centred MONO, it’s coming out of left speaker only!
Yes.
How do I locate the “Output Bus settings”?
I believe so. I checked Devices > VST connections and all saying “stereo”. Should I check somewhere else also? The “control room” is disabled and asks if I want to enable it. I haven’t ever used a control room so haven’t yet enabled it (could this be the issue?).
Yes.
Devices/output/connections…
Wrong routing?
You mention you are seeing Stereo for a bus output in VST Connections, but did you then click on the little arrow beside the word “Stereo” to check if the left and right outputs are assigned to different outputs and not both assigned to one???