I've been after a waterproof that I can bundle into a jersey pocket for ages... Hadnt thought of Decathlon, but sure enough they've got jackets for a tenner. Think I'll be picking up one of those next month.
Mine's a bit too big to go in a pocket comfortably but I wear a backpack all the time anyway so it makes no difference to me. What do you do for pump, tools etc then if you don't wear a bag?