The product you are referring to is probably multifoil. This is a very thin product (perhaps 20mm thick) with multiple layers of foil and quilt that purports to have an insulation value equal to much thicker products. It is often used in loft conversions.
The issue you will have is that you do not want the insulation to take up more space than necessary to insulate the side-walls of your dormer, and any area under the window that has a vertical face to the outside.
In the loft area of the dormer, the company should have insulated that with whatever product they used 4 years ago - go and check. If done with fibreglass, you should be stuffing as much of the material as you have space for (within reason) - at least 300mm. Fibreglass is cheap (compared to rigid foam or multifoil) but it is not so good an insulator for the same width. However where space is not a premium (and lofts are a good example) it is still the best value.
Here is a website run by the multifoil industry that may help you - YBS and Actis are 2 of the manufacturers.
http://www.confederationmultifoil.org/