Well, the way I design my seams does mean that they don't really leak under reasonable load, which is nice.
The thread is a polycotton, and the cotton fibres will swell *slightly*, but frankly I don't believe the story that some threads will *really* swell up enough to seal the needle holes.
So I seal the seams with silicone anyhow!
There's a reason of course. Imminent thunderstorm late in the evening, with severe winds building up. We made it to a known flat open area where falling (big) trees would not hit us (serious concern here). I hastily started to put my new (MYOG) tent up as the rain came down. Sue got inside quickly while I looked after the guys. By the time I got inside we had an inch of water on the ground and the bathtub groundsheet was floating. Yeah, floating. Kinda amusing.
Well, that was OK, but as the storm continued I realised that the ridgeline was very slowly dripping. AARRGGHH! Traumatic! Actually, the amount was not serious, and storms like that one are always short-lived, but ever since then I have been a shade fussy about leaks ...
Rasant is by Amann of Germany, so it should be available in the UK.