Thanks, yeah downloaded it from the MS website last night :-)
PCS should probably update their advice then, to say something like "if you feel confident, we recommend a clean install."
I've checked several threads here on the PCS forums, and most users say that if you buy without an OS, you will need to do a full, clean install.
So I've prepared a USB stick with Win10, and I've got my license key from my previous laptop (which was OEM).
But the confusion is, now that I have...