Thanks guys, my issue is damage to the SMPS Fet in a 24V environment. I've since found the problem to be related to Transient Voltages. I have re-designed all the input stuff, up to and including the SMPS fet(was rated 40V as I didn't understand Transients when I built it), to be rated at the maximum clamping voltage and have chosen 2 x 36V 3000W TVS diodes to be safe. Thanks for your help guys.
Ohh the SMPS just switches down to 12V for other lower voltage bits and pieces, nothing fancy.