This problem was eventually solved, but I am posting the following summary in case the fault sequence and conclusion might assist anyone else. I would also be grateful for any comments on Canon's response as detailed below.
The original fault symptom was that prints very suddenly began to show a cyan cast; no error messages were reported. A nozzle check showed one whole contiguous block of missing Photo Magenta nozzles, precisely 50% of the total for that ink on the Left head, with all other nozzles on both heads perfect. On the dealer's advice, this head was replaced, which reinstated the missing nozzle block, but the printer then returned a -2F43 error and would not perform a printhead adjustment.
Canon's engineer attended and after examining the original nozzle check printout concluded that this was not the typical signature of a failing head and that it was possible that the original left head may not itself have been faulty. He also noted repeated head overheating (-403E), -2F43, and other miscellaneous errors which had not been present previously and which he was unable to clear, despite both head checks showing 100% perfect nozzles.
Having ordered additional components and requested that I also source a further (Right) replacement head in case this was required, he subsequently returned and replaced the head carriage and relay board, and then also the motherboard, which did not clear the repeated errors.
He then additionally replaced the Right head, with no improvement.
As a final measure, he eventually replaced the flexible data cable between the motherboard and the print heads, at which point all the errors immediately ceased and the printer initialised normally.
Both original heads were out of (time) warranty at the time of replacement at 600 days but had logged only 171,000 Mdot (Left) and 136,000 Mdot (Right) total ink usage, well within rated throughput. The total consumables cost of the service included the two new print heads, the remaining ink in some eight cartridges previously showing 40% full, partial consumption of the other four, and 60% of the capacity of a new maintenance tank.
It appeared that the entire fault sequence was most probably due to the faulty data cable in the first instance, as evidenced by the complete cessation of all errors and the resumption of normal operation immediately upon replacement of the cable. It is very probable that the second (Right) head replacement and much of the consequent ink wastage - the head was removed and replaced several times in the course of the service - was entirely unnecessary, and quite possible also that the original Left head may not have required replacement either.
The major parts were covered by a service contract, but I have requested compensation from Canon in consideration of the excessive ink wastage during the service and also the (unnecessary) replacement of at least the Right head, given the low throughput and otherwise perfect condition of both heads. So far they have responded with an offer of 3 ink carts + a maintenance tank, which I consider is probably reasonable in respect of the ink wastage, but are resisting any concession on the print head(s). The gist of their comment on the latter was that I now have two new heads with a new 12-month warranty, so should be happy with that.
I need to respond to Canon's initial offer tomorrow and would be grateful if anyone has any thoughts on the matter.