Canon Pixma MP780 Problems 'Wrong cartridge' message

Old Strebuggart

My Pixma MP780 has suddenly started flashing the error message 'wrong
cartridge' and won't print. It can't be the wrong cartridge because only one
fits! I have been using Hobbicolors refill kits bit for some time and with
no problems. The ink tanks are not 'chipped' so it cant be that and
physically they are identical to the originals. I have tried removing and
reinstalling the tanks and printhead, reinstalling the driver, disconnecting
from power for long periods. Sometimes it works OK for a short while but
then the problem returns.

I am trying to avoid expensive repairs by asking for advice on this group. I
could try a new printhead but they cost around £60 and if it doesn't work
then its money wasted. I like the printer it refills easily and successfully
so don't want to scrap it after only 2 years use.

Can anyone help? Similar problems? What fixed it?

Thanks