The other problem might be the head cleaning pad. This gets full over time and the error could be that the pad is full - or at least the counter says it is full.

There are ways to reset the counter, but if the pad is really full, as opposed to the Canon counter conservatively telling you that you should get a new pad, then it could overflow and damage the circuitry.

And it could be that the Mac driver doesn't have an exact message for the pad - and a new pad is probably part of the new print head set.

As well as the error message there is an amber light flashing on the front - the number of times it flashes also tells you what you issue is. Mine flashes 5 times, which according to the service manual means the print head isn't recognised/compatible. There is a sequence of flashes that also indicates the head cleaning pad is full though, but mine isn't doing that. So it does seem to be the print head in this case. But thanks, I didn't know about the pad getting full until you said. Like you say, there is a way to reset this in the service manual - which I found online.

Will see if I can get some isopropyl alcohol from Maplins

If hammer time is the end result, I understand there are cash back offers on some new Canon printers at the moment. *innocent face*

If the ink cleaning pad is actually full then it will unfortunately be new printer time unless you can find a service guide and buy the part, as getting it serviced is simply uneconomical compared with buying a new printer - I know because this recently happened to my much more expensive MP980.

I've been able to coax a little more life out of mine by completely re-assembling the ink cartridge and print-head unit and then applying the counter reset from service mode, which has banished the error messages and allowed me to print. However I doubt it's going to be a long-term solution.