OK, this works for installing any printer to an x64 Win 7 machine, where the printer is attached to an XP machine (and presumably other OS versions).
I have to credit Barkdust45 on howtogeek.com for the vast majority of it.
The only extra bit relates to the use of Windows Update. You can't seem to use the downloaded LaserJet INF file - but good news - there's a simple way around it. When you have the dialog box listing the printers, you can hit the Windows Update button, and it will refresh the list of supported printers, and the LaserJet 6P is on the list of HP printers. Installing it will take the driver from the Internet.
The secret is based on creating a local port on your machine, for a local printer, but using the network...