Just to update anyone interested.
I received the new CANON print-head #QY6-0049-000 ordered through
eparts on Tuesday 05/04, this morning Thursday 07/04. Fitted the new print-head, refitted the 5 used ink cartridges, all of which took about 2 minutes. Then waited with bated breath and much trepidation whilst the CANON PIXMA MP780 digested everything. After several minutes, it performed an automatic head-alignment, then printed a test page. After which I switched it off, then back on. Worryingly, the amber flashing light then came back on, but just for a few moments.
I've since used it to print, scan and fax normally, without problem. And "touch-wood", whatever
desperate fiddling I got upto in both "service" and "factory" modes doesn't appear to have affected anything...?! But my advice to anyone desperately seeking simple solutions or work-arounds on the www as I did is: beware?! I basically tried almost everything, suggested by anyone who could type, in numerous forums etc. I was in fact desperate (remembering the HP multi-function I spent €450 on back in 2003/4 which worked for about 2 years) and then became an extremely bulky paper-weight...
I'm so happy that I took the decision (on the advice of the CANON technician) to spend €75 on the new print-head which appears to have resolved the problem. I wouldn't have bothered returning such an old printer to CANON for service and it would have ended up on the pavement* just like my old HP multifunction printer all those years ago. (* you call the council here the day before, they come and collect the appliance deposited on the pavement the next morning).
I was just looking through my old invoices. I bought my CANON PIXMA MP780 multifunction back in 2006 for €179.40 inc. VAT (reduced 50% from RRP then). So it's already almost 5 years old and was until just recently, completely reliable. All it needed was a new print-head apparently. I'm not sure anyone else here would agree, but I much prefer spending €75 on a worn-out part on a very reliable machine which might last another 5 years, comfortable with, know how to use etc., compared to buying a new machine...?!