Light it Up: Install PIR (motion sensor) LED lighting on your shed and the rear of your house. It removes the "cover of darkness" that thieves rely on in garden alleys. 

Defensive Planting: Use "nature’s barbed wire"—planting thorny shrubs like Berberis or Hawthorn around the perimeter of your shed to make access painful and noisy.

Secure the Hinge: Use coach bolts rather than standard screws on shed door hinges. This prevents thieves from simply unscrewing the entire door to bypass a padlock. 

Anchor Your Gear: Use a ground anchor and a heavy-duty chain to lock lawnmowers, bikes, and power tools together inside the shed. 

Obscure the View: Use privacy film or a simple curtain on shed windows so thieves cannot "window shop" your expensive tools from outside.