Ah, repeat posts. In which case...
"The GPS only flashes when it's receiving a signal, well on the way to obtaining a lock. If it's downstairs indoors, away from any windows, it won't flash."
I can see one of mine not flashing at this very moment!
I've spent many an hour trying to get my GPS to flash. When attached to the unit and downstairs, even near my window, it doesn't flash. The signal even by the downstairs window is very patchy, maybe suffering interference from my TV/AV gear. Upstairs in my paper house, it seems to work even when away from the window, though gets more satellites when near the window. I also get more ADS-B near the window. I don't receive EGNOS even next to a window - my house is east/west facing, I really need south facing for EGNOS but my next door neighbour's house is in the way! Might try it out in the garden in a mo.
For consistency, always try it outdoors.