One British gun fired on another British gun, the second British gun having been "borrowed" from the ship of the first.

No it did not.
The one pounder was at street level with several big building between it and Helga.
Helga could not see it and could not fire on it because of all the buildings in the way.

Similarly, the one pounder shells fired at a high angle at the flag pole, would land about 2 miles beyond the Liffey.
Helga dd not report coming under fire or returning fire.
It never happened.