Canon Pixma iP 4850 smear problem

[The Yellow Dart]

Hi, I've been printing some cards on my Canon Pixma iP 4850. The card paper is 300 gsm and I printed out quite a few successfully untli it started smearing the bottom left cromer of the print (top right corner of the card as it goes into the printer when you look at the printer from the front)

I've tried all the maintenance things (cleaning, deep cleaning, bottom plate cleaning) and have also taken out the ink head and given it a thorough clean. But the problem is still happening. I've attached a pic of the problem.

Anyone got any ideas as to what might be causing it?

Thanks