All fixed now. Arturia provided me with an activation code for my V Collection 3 which I installed on a spare eLicenser USB stick. I then launched Jup-8 V2 (from V Collection 4), imported the "problem" file, plugged in the spare eLicenser, then ran Preset Decoder.exe and selected the relevant file/project to decode. Perfect. The resulting new de-encrypted file can now be converted for use in Jup-8 V3 using the V Collection 5 preset converter.
Thanks LBH