The first problem is that the Laserjet 6P only has a parallel port, because it was made before USB was a thing, and the xw6600 has no parallel port. I tried using a USB to parallel converter, but I couldn't make that work. So I went on Ebay, and bought a PCI parallel port, cost £2.
I put that into the PCI slot on the computer, and restarted it. Then I clicked on "printers" in the settings tool, and it came up, but didn't show the HP printer.
Google to the rescue.
First, I used lspci -v to find out a plethors of information about all the PCI devices. From this, I narrowed down to:
01:09.0 Parallel controller: Device 1c00:2170 (rev 0f) (prog-if 01 [BiDir])
Subsystem: Device 1c00:2170
Physical Slot: 6
Flags: medium devsel, IRQ 5
I/O ports at 2000 [size=8]
I/O ports at 2008 [size=8]
Using that information, I crafted the following:
insmod parport_pc.ko.xz io=0x2000,0x2008 irq=5
And that did the trick. The HP Laserjet showed up in the list, it told linux what breed of printer it was, linux found the relevant driver and installed it, and now I can use my 20 year old Laserjet!