Thanks SD - yes!
After soaking overnight, I cleaned, dried and refitted the head and cartridges and the printer is now working fine. In the course of all this I've used up three Canon cartridges - £50. Now that I've discovered how easy it is to remove and clean the head, I'll go - as frequently advised here - from now on for Canon compatible cartridges at a third of the cost. Should this result in a printing problem, I now know how to clean the head!
Thanks for all your comments.