OK, We have tried neat bleach, it works ok-ish, and kills pretty quickly, but the weeds come back as strong as ever and thank you for the drink.
We were going back to Round-up but as it's now 65 euros for 5ltrs thats a non starter, so we have tried Glifosato at a much more reasonable 39 euros, and although it still takes 3-4 days to work, we think it is better than the Round-up.
We bought from Sol D'Ebre in Tortosa.