The Nikwax Tech Wash® in 1 litre is a wash-in cleaner for waterproof fabric and clothing which safely cleans and removes detergents. It's ideal for technical and high activity clothing as well as waterproof items such as tents, rucksacks and camera bags. This cleaning fluid can easily be used in washing machines or by hand. For stubborn stains, apply neat Tech Wash® to the fabric with a sponge or brush before washing. Using Tech Wash® revitalises breathability and water-repellency of clothing for extreme comfort when you're in the outdoors. It also prolongs the life of outdoor gear and maximizes performance. The cleaner is great for clothing used when participating in extreme activities. The Nikwax TX.Direct® in 1 litre is a wash-in waterproofer for clothing, producing breathable waterproof clothing that is comfortable to wear. The wash-in fluid keeps you dry whilst you're out and about in wet conditions. It's easy to use too, simply use in a washing machine or by hand. For best results, remove non-washable parts and put items in the wash with Nikwax Tech Wash® beforehand. The waterproofer solution is ideal for clothing with drop liners, Gore-Tex® and event®.

Cheapest I can see online from a main retailer by a long margin. The twin packs are at around £19+ on eBay

Previous threads have offered mixed comments on performance - I have never had an issue. A low heat tumble dry after the proofing wash is the key I think. Link to manufacturers instructions (more detailed than the instructions on the bottle) in below link

Nikwax Blog

Edit: Also same price on Amazon for Prime Members (thanks Broxy for the link)