Mike,

There is one other
signal present which is within 0.1Hz of my frequency and has some of the
characteristics of my signal (dots and dashes made from 30s elements with
gaps in between), but not sure. Will do a comparison in the morning

between

my local Argo monitoring and the W1TAG Argo.

I believe the signal you see is a Loran line from Newfoundland on 135921.60.
I can't see any sign of your signal in the captures, but please do go
through them!

I will leave the capture running overnight in case there are any early
risers who may want to try the morning path.

John Andrews, W1TAG