On the outbreak of war, the security forces were concerned by the visibility of Churchill's country house Chartwell, its high topographical location and the fact it was south of London making it an easy target for German bombers returning from raids on the capital. The Prime Minister's official retreat of Chequers was also deemed vulnerable to attack. Churchill had use of the Paddock bunker in Neasden, but only used it on one occasion for a cabinet meeting, before returning to his Cabinet War Room bunker in Whitehall. However, this created additional difficulties on clear nights when a full moon was predicted (or as Churchill romantically termed it 'When the Moon is High') - so the authorities looked for an alternate site north of London. Tree offered Churchill use of Ditchley, which thanks to its tree coverage and no visible access road made it an ideal site which Churchill was happy with. Churchill first went to Ditchley in lieu of Chequers on 9 November 1940, accompanied by Clementine and his daughter Mary. By late 1942, security measures at Chequers had improved, notably including covering the road with turf. The last weekend Churchill attended Ditchley as his official residence was Tree's birthday on 26 September 1942. Churchill's last visit was for lunch in 1943.