Thanks again Arthur, I am always keen to learn. I will have to look very closely again at the lid of the reservoir as it does seem to have an inner central part inside the actual lid.
Webike Japan, the suppliers have replied already. They have apparently contacted the manufacturers and they have said there should be a small air duct or small hole in the cap. I have checked and there isn't one so this looks like the cause. Webike have asked some other questions and I have replied, so that is positive.
