Thanks for this very useful and prompt reply, I have now finally had a chance to look closely at the printer. I removed each cart in turn but could see no evidence of the contamination mentioned and have now replaced the PM cart with a new one and all is well.
If I get any new insight into what...
I realise from a quick web search that a lot has been written about clogged heads, I hope this therefore does not bore you all.
I had to ship my canon i9900 from the US to the UK and have just got it going again at this end. Total inactive time about 7 weeks. First print out is extremely flat...