I bought a few sizes (EBay – medical/food supply) and cut thin o-rings. The problem is that the silicon tube wall is around 1.5mm thick and would not go into the fitting. I successfully made it work by stretching a smaller bore, but it was a fiddle. Much easier are the 1mm thick o-rings from Reeves, and they are only about 30p. I used two at each end with a touch of silicon grease. I also agree with the PTFE tape and Foliac (steam seal) solution.
Norm.