I must have mislead you, probably my fault.

The current set up is:

Two old G4 towers OS9. Both are ethernet wired into the router.
Two new iMacs OSX. One is ethernet wired into the router the other is wireless.

All four share the printer seamlessly. Despite their antiquity the old G4 OS9 towers have been ethernet-connectivity for years.

Doing some more research, surely all I need is an USB to Ethernet (RJ45) adaptor. It's a little box. I'd plug the new printer's USB cable into one slot, and plug the current printer Ethernet cable into the other slot. (The other end of the printer Ethernent cable remains connected to the router.)

Could it possibly be as simple as that? I sure hope so. The additional spend will be a mere £8 or so. There's loads of them on Amazon.

I've emailed Brother to ask them too.