Invest in a Buffalo windshirt, or a similar Pertex garment.
Depending on the weave of the Pertex, as there's Pertex 5 and heavier Pertex 6, it will provide maximum wind protection, for a fairly modest outlay.
The Buffalo windshirt i own, has been up Pen Y fan so many times, ive lost count, and its survived over a dozen or more Challenge walking events, both in Wales and England.
When its particularly chilly, and the sun is out, i like to wear it over a thermal top, and it just soldiers on, year after year, providing great insulation against a keen wind.
Ive owned two Buffalo windshirts for the best part of thirty years or more, and it takes a considerable number of years for the Pertex to show any significant signs of ware or age.
A windshirt, can be packed down into a very small size, and dries out incredibly quickly.
Over the years, Ive bought quite a bit of good quality walking kit, but the only items i still own, that have been with me from my early days as a walker, is a Buffalo windshirt, and a lovely Berghaus Dart rucksack.
The rucksack was purchased in Chrickhowell Adventure gear, sometime in either 1982 or 83.
Made of Cordura, its got years of life left in it, and even the windshirt is of 1980s vintage.
Its an item of clothing that can be used both in Summer and Winter, and its ability to pack down into a very small lightweight package, makes it ideal for storage inside your sac, on the off chance the weather dicatates its use.
Get yourself a Buffalo, it will be money very well spent.