Yes Compilla, this thread was relevant. I checked the Hidden Menu, as shown by MacDaddy and all of the valves were on.
I investigated further and removed the pipe from the yellow sub tank at the print head (when the machine was powered off) and the ink disappeared back along the tube (I guess into the sub tank?) and the yellow line is now clear with no ink. I re started the 1604 and the SubTankLo message is gone, but the yellow ink supply line is now empty.
I read in some other thread about using a syringe to draw ink into the system. Could this be useful? . . . and if so, where should I apply the vacuum?
PS As a point of information, the dealer here in Ireland from whom I purchased the machine has been liquidated - I have contacted another Mutoh dealer, who have sent me a service kit - There is no official Mutoh Tech person here in this country (that I can find anyway), but I am waiting on a 'wide format' tech person to have a look at the machine, when he has time - hopefully soon!