When I first saw your post I didn't know what it was. But I may have stumbled across the answer. Lots of flies have a black stripe on a reddish background, particularly tachinids, but they don't all have those little pale triangles. I wonder if it's a Hybomitra species, which are a type of horsefly. Is that any help?
"A medium-large horsefly that is highly variable in appearance, with some specimens having extensive red at the base of the abdomen whilst others are all-black. Females and males have very long hair fringes on the anterior and posterior surfaces of the mid tibiae. Males have a long posterior hair fringe on the hind tibiae which helps distinguish them from males of the scarce bog-loving H. lurida. Female H. bimaculata have three dark stripes across the eyes.
H. bimaculata inhabits a variety of habitats, including a range of wetlands and open-structured wet woodland. The males can sometimes be observed hovering, like strange-looking blackish hoverflies. The predatory larvae develop in wet soil, wood detritus and even waterbird nests."