After reading the above comments it became clear to me that everybody was talking about "targets" so I thought that at least I should try to find out what one was and how it is used. While hunting I found the following quote about scanning color negatives from a book "Real World Color Management" by Fraser, Bunting and Murphy on another forum, which suggests there are several obstacles, namely (to quote)
* Nobody makes a color-negative scanning target.
* Unless you like orange, inverted images, you don't want to reproduce what's
on the film.
* The orange mask on negatives varies so much with exposure that, even if
someone did make negative targets, they'd only work if your negatives were
exposed the same way the target was.
I've seen flows that make a valiant effort by printing from the negative scan
and trying to profile THAT, if you want to try it. Bon chance.
After reflecting on this for a while I looked up my Imacon Flextight Precision II scanner manual and found instructions for white balance calibration of reflective images with the recommendation that this should be done every time the tubes are changed. So I did it (with a white piece of A4 paper and no plastic sleeve ) though of course! this seems to have made no difference to the negative scans (which are done by a different tube than the one used for reflective scans). Perhaps I also managed to completely screw up the calibration of my scanner for reflective scans but that is another story and no big deal since I never do reflective scans anyway.
Yesterday I found a Photoshop plug in by ADG that adds to filters in Photoshop a sexy plug-in with a color temperature slider to adjust the color temperature of TIFF and other files in Photoshop. When I used this plug-in to reduce the color temperature of the image from negative that I had scanned with the new tubes (5400K) by 400K the colors did actually look more similar to the colors of the image from the same negative scanned with the original tubes (5000K). But strangely enough both images appeared to be much too yellow (even though I had carefully calibrated my monitor with a spyder) to begin with, indicating perhaps that the color temperature was too low on both scans (could this be caused by old tubes).
Consequently, the most "natural" results that I have been able to get so far from either negative scan (old 500K tubes or new 5400K tubes) were not by adjusting the color temperature using ADG's Photoshop plugin but by using Photoshop CS3 IMAGE > ADJUST > photo filters > 80 or LB cooling filter. I guess this is just a different way to simulate reduction of color temperature via adjustment of curves and levels than the ADG color temperature plug-in rather than actual filters - but at least they provide a standardised profile that I can apply to every image I scan as a starting point for adjusting the colors.
As a result I thought that I had found a way out of the jungle - until of course I looked carefully at the very interesting chart Czyorni posted and read these conclusions, which seem alarmingly logical even to me:
"It's hard to predict what colors will be affected in case of your light source, and the distorsion probably may be too complicated to correct it with some simple Photoshop curves. The simpliest solution is to buy a scanning target that contains many color patches of known colorimetric values, and create profiles, that will contain correction tables. The only problem is, that negatives were not stable and too tricky to profile, so negative targets were not very common - it's only easy to buy targets on transparencies and reflectives."
One steps forwards, two steps backwards...
Well at least I now know more than I did before but perhaps it is true that a little knowledge is dangerous.