Hi Graeme , is the auto shut off valve diaphragm operated inside ? It could be that the diaphragm inside has split and you could be getting carry over which would effectively bi pass the RO membrane
We have a winner!
I removed the membrane output pipe from the auto shut-off valve, and ran the system. Water was pouring out of the hole left in the auto shut-off (pretty sure that shows the membrane was ruptured). So, I removed that altogether, and tested the membrane output. The water now runs slowly, and the TDS output from the membrane is 002 ppm. Hooray!
I've now spoken to Vyair, and Phil told me he's seen this before. It can be caused by sudden pressure into the system, which is sometime created if a dishwasher or washing machine is connected to the same outlet feeding the RODI. That's the case for me
They are going to send me an auto shut-off for free, since I paid for parts that weren't at fault.
What I'm not sure about is, how do I prevent this happening again?