Just did a search on RC and found out other people who use clear silicone have the yellowing. In their case they used Silicone II though, so I guess it doesn't matter about the difference between 1 & 2, if it's clear, you'll notice the colour. I've always used black in the past so maybe it's just a case of never noticing that there was discolouration.
Odd though, that the clear silicone holding the tanks together don't discolor. Different silicone? Or different curing method (more time??) ?
Anyhow would still like to know where's a good place to get the 1200 or the 128rlt or whatever it's called ... so I know for next time. Thanks!