Weak and crumbling walls in Victorian prisons are hampering attempts to halt drones from delivering drugs and weapons to inmates. Plans to install tougher netting and window grilles to stop drones from entering have been hampered because the walls have been unable to take the extra weight, prison governors said. Recent attempts to fix anti-drone netting at HMP Pentonville, the Victorian prison in north London, were stalled after they found that the bricks were too soft, sources have said. Charlie Taylor, the chief inspector of prisons for England and Wales , said last month that the Prison Service had “ceded the airspace above many of our prisons to serious organised crime”, resulting in a “national security threat”. The number of incidents at prisons involving drones has risen by more than 1,000% over four years, with gang members able to fly packages carried by drones direct to cell windows. The packages are then retrieved by inmates with a hook.…