Thanks to Atlex and Bullfrog for your replies.
We have been printing 'purge prints' of the problem colour - Blue - and they are not true to colour but not as far off as the nozzle check pattern suggests they should be.
This problem has been developing for a while and we think it is partly our fault because we have not been using the printer enough (three house moves in two years hasn't helped). When the problem started the nozzle pattern was only slightly off but we were getting the 'Clean RH Printhead' error message. As we don't use that colour a great deal we pressed on and for a while the error message went away. Then it came back with a 'Replace Maintenance Cartridge' message, and we did that. And we bought a new print head which we installed a week or so ago. We have also changed the Blue ink cartridge and, on the advice of a local supplier, we have made some attempt to clean the various contacts. None of this has made any difference.
Maintenance for us is a problem as we live in the sticks in France a long way from the nearest city big enough to support a competent technician. And with a four year old printer it might work out cheaper to buy a replacement. Canon, in the UK at least, don't want to discuss problems with a printer of this age and send us to Oce who apparently made them then and have some quite ambitious ideas about telephone helpline costs. It's a pity because the problem makes surprisingly little difference to most of our output.
In case it is of interest we have posted the latest nozzle check on our at the moment redundant blog: http://bowaterphoto.blogspot.fr/