Sorry I think we've gone slightly onto different wavelengths; let me rephrase my previous query.
The Arduino's take a recommended 7-12V (and specced 6V minimum) due to their own 5V regulator, and my power supply outputs 24V.
I'd be more than happy to use a 5V switching regulator to step 24V all the way down to 5V; probably something like this http://uk.rs-online.com/web/p/buck-converters/0398477/.
But is it possible to do this without replacing the regulator on (or otherwise modifying) the Arduino board? (Given the required min 6V input?)