Resolved: Inconsistency in the installer database.
Not even the restart fixed it, which we all know is not very often the case!
Having spent a few frantic hours googling and trying out potential solutions, I finally came across the Adobe Creative Suite Cleaner. A quick run of the tool and finally the install worked!
It's also worth pointing out, that event though the clean up tool may not be able to find the installation, click the cleanup button anyway. Now restart the trial installer, and you should find it will install as expected.