Thanks for all your feedback. I guess a simple resistor isn't going to cut it
The reason I'm using this particular driver is that it is also powering a number of high-power LEDs. The Dialight is primarily designed for this task and I was hoping that I could also use it to power the Uno too.
I could add a separate power supply for the Arduino - the downside being extra cost and I'm a bit tight for space. And being in the UK we can't get one for $25.0
So I'm looking at simple but expensive solution which is a 9vDC transformer - http://uk.farnell.com/xp-power/ecl05us09-t/psu-5w-open-frame-9vdc-0-55a/dp/1765276
The cheaper but trickier option of using a switching voltage regulator (as recommended) and trying to build a suitable circuit. But I have no experience in this area so I'd very very grateful for any advice, recommendations or circuit diagrams.
Many thanks guys,
P.S. I also like the idea of cannibalising a car-charger