I have installed a complete set of four 'compatible' ink cartridges from
Cartridge Concept in Scotland.
I can print normal text, e.g. from a Word document, OK, but if I print
anything that has dark areas, or even a word table with line width set to
1.5 pt. the printing almost disappears after the first few passes of the
head, leaving a few faint streaks and many large blobs of black ink.
If I make the same table coloured instead of black it prints perfectly. I
remember having the same problem when I tried refilling genuine cartridges.
Is there anything I can do to fix the problem, other than buying expensive
Canon cartridges?

Thanks,

Kevin

Go out and purchase a set of good cartridges and refill them with
compatible ink. Make sure it is factory spec ink, that is, it meets
factory spec. Don't rely on on verbal assurances that it will work,
make sure it is in writing and you purchase from someone you could
complain about and at least get your money back.