Revised opinion - this is a MALE intruder that fights the female of the existing pair.

[edit 19th august] An established pairing is challenged by a stronger female (I originally thought - now sure it was a male) newcomer, resulting in a full on confrontation between the two female birds. The punchline is that the winner - instead of staying to start a family - disappeared the next day not to be seen again - until mid-summer when she (he) returns to cause trouble..... (see “Fiesty Chick“ vid.)