I finally discovered that the return tube from the reservoir container, was curving back upwards within the container, such that it was above the fluid level. This resulted in no return of fluid and constant loss via the overflow.
I have fixed the problem by fitting a suitably shaped metal tube to the end of the return tube, so that the inlet remains at the bottom of the container.
This would appear to be a fault built in at manufacture. The shape of the container means that the lower fluid level is of little capacity, and the tube easily rises above above what is available in the bottom section.
This could be a tip worth publicising.
