Very interesting, that would be surely be BE's longest routing on the Q400 by far. Great circle mapper shows it is 922nm, which is further than SOU-FAO! (883nm)
BE did some ad-hoc pax flights on the Q400 BHX-ATH over Christmas to take families out to crews working out there. They also move Q400s between the UK and Greece direct. The longest non-stop sector is EXT-ATH at 1394nm Great Circle although the actual routeing will have been slightly longer.