OK so got the 303 spray yesterday and sprayed a pair of Decathlon overtrousers that had been washed in detergent to remove any old proofing and rinsed 3 times.
The stuff has solvent in it so it smells a bit like scotchguard aerosol does, needs to be done outside. Then I've dried them for 24 hours which is the max time apparently needed to cure the coating.
They repel very well indeed. The tap test shows it beading like mercury. A 10 m inutes shower test fails to wet it and even rubbing the water in with fingers is very difficult. Any water that sticks is easily shaken off. I've had new jackets be less resistant than this. No sign of any harm to the seam tape from the solvent - and I deliberately soaked parts of the seam with it to test for this. So far it's obviously superior to Nikwax and Grangers I've used previously. It will need a proper outdoor test in British weather to be sure.
As a bit more of a test I sprayed a bit of normal, non-waterproof fabric. I did half in Nikwax Direct spray and the other in the 303 and dried it for 24 hours in a warm spot. I repeatedly sprayed with water, scrunched up the cloth whilst wet and then shook it off. The Nikwax side did OK - water beads up nicely but repeated wetting and the scrunching sees the water starting to get a grip. But the 303 is almost impossible to wet, even though it's just a piece off an old polyester it's acting like it's properly waterproof.
I've attached a photo. It's pretty obvious which side is which.
https://ibb.co/kZXrG5To be honest I'm amazed just how water resistant this bit of polyester has become with the 303. It appears to be actually waterproof to all intents and purposes.