Yes, thanks Mixture. The setup in the '3600 is of four imaging units stacked one above each other (CMYK), then a transfer belt rather than drum, and then the fuser unit is at the top of the lot of 'em. (Actually, the 'imaging unit' is combined with the toner hopper, so quite compact really.)
By using HP's diags which involved swapping a couple of imaging units around (so that the cyan unit thought it was the yellow and vikkyversa) I've proved to my satisfaction that it is the imaging unit itself which is at fault. Just bad luck to have two successive cyan ones which were both less than perfect. Recycled ones rather than OEM - but at £30+ as against £100+... that's the way it must go.
And I've got an inkjet photo printer if quality really matters.
A bit of thread drift, but interesting...
While I was trawling through the printer menus I found 'Print RGB Samples' which proved to be 20 pages of printed samples, each with 72 different colour swatches on them. Colour matching has been a bit trial-and-error to date, since I couldn't find an algorithm to match screen colours with printer colours. Now at least I have a 1440-colour palate (?sp?) to choose from. May be useful some way down the pike...