^^^^
In my opinion, all other lights should go off when DRL's are on.
I think I would agree with you, Robin - that would be my preference too.
On saying that, I do not think that there would be anything illegal about having the DRLs dim when other lights are switched on since the law only seems to say that it is not acceptable to dazzle others. I am certainly less happy with the DRLs being on full power at night even with headlamps lit - could be a bit 'iffy' legally and I am not comfortable with it morally....
....which brings me to Update #2....
I have been able to get the DRLs to dim with the sidelights. So Picture 5 in my previous post showing sidelights on has the DRLs operating but with the brightness shown in Picture 6.
This is done by wiring the yellow DRL wire into the grey/black wire going into position 10 on the wiring loom for the car lights. It seems that things behave differently when the headlamp is attached to the wiring connector which was why I was getting misleading readings with my multimeter when testing voltages on the connector only (without lamp unit attached).
However, I can not find any way of getting these DRLs to automatically go off when either the side lights or headlights are switched on - they only dim. There may be something I haven't discovered yet but I don't think so.
Interestingly Philips seem to have a competitor product to the DRLs that Carl and I have which look like they
do switch off when other lights are on. However, they also seem to have an integrated control unit which may explain the clever stuff (and the price tag!). Here is a link to the installation book for them which also gives some useful information on what is legal in terms of DRLs and their positioning on a car
[pdf]http://www.powerbulbs.com/drl_guide_all_languages_final.pdf[/pdf]