REMAP Mcode seemingly ignored

Hi.
I've added a custom M199 code to enable my  spindle-index-enable signal, and this runs fine. (Can see the LED light up in halshow when run.)
So now I'd like to build it into my M19, so the M99 runs just before the M19 is called.

In the [RS274NGC] section of my INI, I have put
REMAP=M19  modalgroup=7  argspec=RQP ngc=custom_M19
my custom_R19.ngc file contains:-
( Custom spindle orientation ngc file)
( M199 raise spindle-index-enable signal)
o<pre_M19>sub
M5
M199
G4 P2
M19 R#<r> Q#<q> P#<p>
o<pre_M19>endsub

The G4 pause is just so I can be sure that the signal is enabled in halshow gui.

I'm pretty sure the file is being read at Linuxcnc fire up, as when I remove the file from the macros folder, the startup parser errors with missing file.

After I home the machine and run the MDI command say M19 R20, the M199 and the pause are not being executed.

I feel I have missed something really obvious..  be grateful for a steer. Thanks

When working with multiple m codes, i always use Queue Buster before each m-code.
eg.

M66 E0 L0
M169 P9

M66 E0 L0
M169 P6
 

I think that's a good thought, however, I've found M19 is not available to directly remap according to this .
14.1. Existing codes which can be remapped
The current set of existing codes open to redefinition is:
Tx (Prepare)
M6 (Change tool)
M61 (Set tool number)
M0 (pause a running program temporarily)
M1 (pause a running program temporarily if the optional stop switch is on)
M60 (exchange pallet shuttles and then pause a running program temporarily)
S (set spindle speed)
F (set feed)