What a can of worms! To rectify the above mistake, I got a spare vlm cart and filled it with the right ink and did some clean cycles. There was some improvement up to a point, so I deduced that there was probably still residual vm ink in the vlm line that was going to take an age to completely remove.
So I started selective purging of the vlm, and what happened? Well, just before that line stopped working completely the color suddenly went a much deeper tone after I'd printed off 6 or 7 A4 pages of what looked like the correct ink color! Very strange. I can only think that some hidden trapped reservoir of vm ink from my mistake suddenly made it through to the paper.
Anyway, now that line prints nothing, despite many cleaning cycles. Nozzle checks give only a couple of millimeters of print at most. I've tried cleaning the print head as per the Inkjetmall maintenance videos but with no difference. I'm thinking two things: either I have a faulty cart, (which seems unlikely and which would be very unlucky), or vm ink particles have created a really stubborn blockage, in which case perhaps I need to selectively purge that line with Piezo flush in a fresh cart, possibly using the printhead unblocking facility from 2manuals.com
Before someone asks, yes, the carts are more than half full. None have been allowed to run empty, and the air holes are open. I've had this printer maybe 3 years now with no previous issues and the printer has never been professionally overhauled. All issues stem from that one wrong color refill mistake, and boy how I'm paying for it now!
Any suggestions, anyone?