I was asked by a few more people just what's wrong with the printer...
The most likely fault is an intermittent one with the cable running between the head assembly and the main board.
This causes partial misfirings in the head. In this case a new print head (the 8300 takes two PF-05) would work just fine for a while - over a year for this one until the printer decides that it needs a head again...
There are various tricks in the service menu that you can do, but the main thing is that there is a problem other than the apparent head related one - discussing the 8300 with other users suggests that the printer (from Nov 2010) is an early one that didn't have an updated cable fitted - there was never a formal recall AFAIK and Canon has never publicly acknowledged any issue...
It really is free to take away and has quite reasonable amounts of ink in it. There is a straight through route to the front door from my print room, so apart from two strongish people to actually move it off its stand it shouldn't be too difficult to shift...
I'm told by the house authorities that it will go to the tip if need be... :-( Given the house management is smaller than me, I'm curious how that will actually happen, but the threat has been made :-)