Converting QDF files to CSV for uploading into Quickbooks - how best do I do this?

Have you tried the following reference?

https://quickbooks.intuit.com/learn-support/en-us/convert-data-files/convert-quicken-data-to-quickbo...

How old is your Quicken version?

I was using 2011, but updated to 2020 recently - which is the version that generated the QDF file. I have however, used a convertor to create a QXF file - but Quickbooks requires CSV files, and I can't create this as I can't access the original Quicken (Reckon) application.

Try installing the trial license of QB Desktop 2020 Pro and convert any QDF file available on your local drive. 

I've downloaded and installed QB Desktop 2020 Pro, but it doesn't give me the option to find QDF or QXF files - it's looking for QBM QBB or QBW files only.

Convert your Quicken file using the Conversion Utility in QuickBooks Desktop.

To do this, go to File > Utilities > Convert, then select From Quicken.
Select the Quicken file (.QDF) that you want to convert.
The Conversion Utility will guide you through the rest of the conversion.

Downloaded and ran the Convertor, but when I now try and open the files in Desktop 2020 Pro it asks for a password - and the password we used for the old files doesn't work, and we werent' asked to set a new one. 

Try change your password on the Quicken file and run the conversion again. If the same issue persists, consider purchasing a 3rd party conversion service. Contact us for details.