Embguy was right: the purge unit had failed. Cleaning didn't revive it but a rather expensive replacement purge unit has got the i9950 working properly again. I am grateful to everyone who contributed advice.
However, I am now worried about Scots members discussing the drinkability of household...
None of the window cleaners at my local supermarket contains ammonia. I don't know where to find a cleaner that does. On 1st September Canon UK Technical Support e-mailed me an automated acknowledgement of my request for help, but they haven't yet contacted me with any actual help.
I have to...
I have a different fault with my i9950. A few days ago black became streaky, so I tried first "cleaning" and then "deep cleaning". This did not clear the fault: instead black stopped completely.
I took the head to the local supplier from whom I bought the printer. He has cleaned the head for me...