Thanks for your prompt response. I have just discovered the problem. I decided to put in new cartridges and in so doing I noticed that they had a letter P after the numeric code. For example 6PC which should have been 6C.
They were genuine new Cannon items but the store where I purchased them made a mistake with all three colors and I didn't notice it. I looked back at a previous purchase and I found they had made the same mistake before with one out of the three colors
As my color vision isn't really that great I accepted the test pattern as OK when in fact it isn't
So rule for me in future is to read the package label more carefully