Hi, just wanted to pass on my experience as to how I managed to fix this problem.
The light magenta cartridge would not print despite 5/6 nozzel cleans. It took some time (basically overnight soaking) but I used a bottle of cleaning fluid, injector & flexible tube (Ł10 + del)) supplied by Crab Chemical (see link below). I removed the offending cartridge and purged the nozzel with 0.1ml of cleaner 4 or 5 times throughout the day and left to soak overnight - Bingo it worked!
Hope this info helps others.

http://www.inkjet-service.co.uk/print-head-cleaning.html

doesn't the soaking damages the printhead in some way?

No if you avoid products that are "too agressives" to plastic or cupper