Guvnor, can't remember the TV show you mention. Pathfinders used Lancasters as well as Mosquitos. The Mosquitos flew high and fast ahead of the main bomber stream to drop target markers, but the lead Squadrons of Lancasters were also Pathfinders. In Len Deighton's Bomber, very bad things happen when a Mosquito is shot down and jettisons its target indicators before reaching the intended target.