Voids and debris
I bought a cheap Depstech endoscope from amazon. It's an impressive bit of kit for £30. The camera head is thin enough to fit though a lot of the little holes drilled in the wall for pipes and cables and heating vents. As your mobile phone is the viewing screen. Photos and videos can be taken while rummaging through a nook. For very narrow holes, there is a mirror attachment allowing you to survey the expanse of the cavity. I tried it out on the cavity above the combustion vent in the living room. The installers had neglected to put a sleave on the vent which means the cavity is easily accessible. A correctly insulated cavity should be difficult to see anything due to presence of dense insulation. But here there was a large void above the combustion vent. The picture below shows a reason for the void: A long wooden baton sits in the cavity obliquely and rests against a fish tail walltie. The baton has probably fallen into the cavity when the roof wa...