Led me in the direction of getting a result via UMG, though more complicated that seems necessary - as you have to convert positions and sizes using anchor points to get screen space information.
I’d like a function - GetFinalLocation( FVector2d &outMin, FVector2d &outMax ) or something similar.
These functions might be close (as seen in this thread), but when I printed out their results I didn’t understand that UMG was thinking of my Y value as being -ve from its anchor at the bottom of the screen.
Will revisit that when I next look at the code!