Thank you but turning off the printer and unplugging it, then trying to remove the cartridges was impossible as, even with force, the print head mechanism wouldn't move from the resting position at the far right hand side.
Plugged back in, open door remove cartridges, close door, turn off, unplug, press button for 30 secs, plug back in, (nothing so cant reinstall cartridges) pressed power button, error message straight away, replaced cartridges, still error message.