HF500 Not starting on M3 Code

Hi all!

I'm currently running a Stepcraft 600(2) with a HF500 Spindle, UCCNC as the controller and Fusion 360 as the CAD/CAM (see bottom for example M3 Code). I've have had to resort to manually switching the spindle on and off as the following things occur when the program runs through the M3 code:

1. If the spindle is off, nothing happens and then the dwell time occurs, then the program carries on and if I don't get to the stop button fast enough it'll snap an end mill. The controller box DOES start to flash red quickly, but won't stay solid red. The UCCNC software shows the clockwise rotation button as green.

2. If the spindle is on, it will stop the spindle and carry on, but the same flashing red light at the controller box and 0 shown on the LED screen.

3. I've tried swapping the M3 code to read the speed after the M3 rather than before, which seems to make no difference.

I have checked other threads and tried the same settings for the HF500 as everyone else (see below for screenshots).

FYI the code produced by Fusion 360 (my CAD/CAM program) is:
----------------------
(1)
(Outline and Text)
(T10 D=3.5 CR=0. - ZMIN=-6.15 - flat end mill)
G90
G0 G53 Z0.

(Pocket1)
T10 M6 (flat end mill D=3.5 1FL ALLY CUT)
S13000 M3
G64
G54
G43 H10
and so on....
---------------------

Any help is much appreciated!