Thought I'd just report back on this issue, which is now resolved, as opposed to solved!
Around the time I started posting on here, after spending a very long time going round in circles on the help/support site, I'd sent a general request off to Roxio asking about how I could get a 'support code' to enable me to contact them about my problems. (I eventually did get a reply and link to Corel site which I couldn't enter as it didn't recognise my email, and wouldn't let me register a new account as my email was 'already in their system!')
On finally making contact with the details of the problem, (on 4th) I've thence been communicating with Ralf, emails and eventually a very long live chat remote session (1 am this morning until 3.30 or after!)
He stripped everything out, ran the download and hit exactly the same problem as I did, and concluded it was 'weird.' He has eventually sorted out my problem by very kindly installing Creator NXT instead for me.
So - my problem is resolved, but the cause is still unknown!
Again, many thanks for your time, and also contacting Customer Service on my behalf.