Ok great, thanks Tom.
We'll be wintering ashore, so we don't even benefit from the "warmth" of that Scottish water!
I think I will try to find some kind of "safe" low-level heat producer for the engine room.
Whilst my Dessalater Duo 100 manual is silent on the subject of anti freeze etc, it does very clearly state that the membranes themselves should not be taken below 0C.
I guess if I drain the tube, and production pipe from the membranes that will also drain any residual fresh water in the membranes.
I like your solution of using Steriliser + Glycerin, if this passes through the membranes as you suspect, then it would give protection to the tube.
SY Stella A54-154
Loch Stornoway (Sheltering from a gale!)