My problems with this are now resolved. Simple really! The (extra) Vetus raw water filter was missing the "O" ring from its lid. No vacuum=no cooling water.
The filter is now bypassed and will be skipped this winter when the awful little external strainer-inlet will also be replaced with an internal tubular filter/strainer attached to a "straight-through" skin fitting. I had intended to do this before relaunching but ran out of time.
275/30 "Cascada"