FlyBe Embraers most definitely do not use ADS-B...
The position shown seems to be "where it was" when it was last plotted, as you say, they're over your head when "passing the viaduct" there's a delay involved, they're not ADS-B so it's not (reasonably) recent position information.
Keep using your viaduct as the overhead for the non ADS-B and you'll likely see them then!
I think it's an FR24 thing rather than a specific computer/laptop/tablet thing, I get varying results with this computer compared to an ACER chromebook. You get used to it after a while!