Another update. Removed the printhead and cleaned with a solution of 50:50 distilled water and ammonia based window cleaner. Warmed the solution up and soaked for three minutes. No good. Repeated. This time the test print printed the cyan but then the cyan ceased to work. It's clearly a blockage in the feed from the cyan cartridge. Now soaking overnight. I now have nothing to lose since a new printhead is £75 and a new comparable Canon printer is £63.