New one on me - but the description seems to match !
anoderma applanatum (Pers. ex Wallr.) Pat. Flacher Lackporling Ganoderme aplani, Artists Bracket. Bracket 1090cm across, 560cm wide, 210cm thick, more or less flat, semicircular, hard, corky and glabrous, margin acute; upper surface knobbly and concentrically grooved, covered with a hard wrinkled crust, often pallid, grey-brown, umber or cocoa-coloured. Flesh cinnamon brown, thinner than the tube layer. Taste bitter, smell mushroomy. Tubes 725mm long in each annual layer, brown. Pores 45 per mm, circular, white, bruising brown. Spores brown ornamented, ovoid-ellipsoid, truncate at one end, 6.58.5 x 4.56um, mostly 8 x 5.5um. Hyphal structure trimitic; generative hyphae with clamp-connections but these may be very difficult to demonstrate. Habitat on the trunks of deciduous trees, especially beech, where it causes an intensive white rot. Season all year, perennial. Uncommon but until recently much confused with G. adspersum. Not edible. Distribution, America and Europe.
I'll have to do the bruise test when i get home ! - But Doe's the underside of Fomes also bruise purple ?