I`ve used Jettec compatibles in the past but never had a print head clog. I`d suggest replacing the Black ink cartridge and phoning Jettec first thing on Monday. Actually, try the supplier first, and if they quibble tell them you`re going to ring Jettec. I dont think you`ll have a problem. Do keep the cartridge in case Jettec need it.
Jettec helpline is 0800 0202 488.