Well, I managed to unblock the yellow nozzles by repeatedly running the cartridge clean routine.
Unfortunately, in the process, I managed to use up all of the black ink. So I went out to get a new cartridge.
Printing the alignment page today, I find that the cyan and magenta inks are just about empty as well.
Fortunately, I have a spare colour cartridge here, so I've fitted that along with the new black cartridge.
More fortunately still, Lexmark provide a recycling envelope in the box, for sending your empties back, thus saving the environment.
So I feel miffed at having spent £45 on new cartridges, but slightly better that no fluffy rabbits needed to die for it.
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roger