@T3P3Tony
Thanks for your advice- however I've inserted that command line into the spindle setup file to no effect. (there's no need fo L and Q values) The G-code files I use all have M3 and M5 in the header and post script. Is there anything else I should be doing before looking for a hardware fault?

This is a very simple query which is that I need to find a way of switching the "e1heat" 24v output, which is connected to a relay with flyback diode, to switch a router spindle and dust collector at the beginning and end of a job using the M3 and M5 commands.
I've looked but can't find any guidance specific to this- all the responses appear to be about driving a VFD!