Having digested all of the above comments, so to speak, I have formulated some explanation for the behavior of my system.
I think the blue the previous owner used killed everything and so no gasses were produced and there were no smells to start with .
I stopped using blue after about six months and changed over to Silky but it seems the transition can take longer than I thought. If there is still blue in the system killing the bacteria, but not all of them, leading to some fermentation of the waste and some gas production which is venting along the pipe from the waste tank to the toilet.
The non return valve is probably there to prevent the back flow of liquids but not gases.
Hopefully with each new pump out the gas production will get less and less as a finally get rid of the last of the blue.