We love this stuff: Fabsil Universal Protector. We’ve been plastering it all over our tents in readiness for the new season, and after a little test can confirm it works. We painted half a tent with the Fabsil, left the other half untouched, and then waited for the rain. You could see the water beading on the treated half, whereas it started to soak into the fabric on the untreated side.
But hopefully it won’t be too heavily tested this summer!