- Edited
Staudey it didn't download from a request inside of the VirtualBox program, something about an invalid certificate. Figured it must have been something I had broken and just found a way to make everything work again for me.
Running it as an attached CD drive and installing from there worked fine.
TBH I only encountered the issue on my Plasma machine directly after the update to 7.0.14
There was plenty of Virtual box forum chatter about Virtual box Guest Additions not installing so guessed (perhaps wrongly) that the fault was theirs, not mine.
Only updated guest additions on my Budgie machine today and did not bother trying to download from within the program as I already had the ISO downloaded and knew how to set it up quite painlessly.