OK, thanks for the ideas everyone. I have tried most of them, including cleaning with MUC OFF disc clean I use it after every few rides anyway), although cleaning discs again with meth spirits sounds like a good idea.
Cleaning my removed pads again, and this time also using a bit of sandpaper sounds worth a try too.
Someone (a bike shop that should know better!) suggested that I dab a bit of copper paste on the back face of the pads, to prevent mechanical resonance. I will never do that, because although it sounds like a good idea, such paste will contaminate everything and slowly creep everywhere! (Someone I know did this and learn from the mistake).
Softer Resin pads, yes I have used those before and yes both performance and quietness was very good, except I really do need to keep running sintered pads because: I am slightly big/heavy, plus often carry a modest weight on my back, plus it is winter, plus they last alot longer generally anyway. And also because: I have used Hope sintered pads before, without ANY problems.
Meth spirits and sandpaper it is then guys… thanks!
GBR.