"At first glance it is not obvious how to use this product but a quick Google session reveals all. Contrary to the previous review, these are in fact designed for the cavity size specified - the DPC goes against the back of the outer skin, the insulation bridges the cavity to the specified width, and the clips fix the closer to the side of the inner block skin. If you want to look it up, this product is made by ybsinsulation and is called therma-close. Their website gives details of how to install it. Perhaps Screwfix could attach a copy to the product description here."
"These do what they're supposed to do - building inspector was happy with the design and they fit into 100mm cavities OK. A bit pricey for what they are - simple enough to build your own from components - but if you've only a couple of windows to do, they're just the job."