Foundthe problem by and large- I changed the red background to black and the problem was completely solved with the filtering disabled.

Enabling the filtering works _almost_ perfectly but left a ragged edge for some strange reason. Decided to stick with unfiltered rotation which looks very clean.

I am so pleased.....

And the moral of the story is - always use colour RGB 0,0,0 for yor translucency. Suits me ok, but perhaps would cause issus on most other games for example?