Ran various tests on the installed Lowrance vhf transceiver and, eventually, have taken the view that it is unserviceable with a capital F. Changed out for an ICOM unit with an internal GPS. This unit is then, for the moment, not networked to other elements of the nav/com system. Probably the right way to go; I may loose some potential features (integration with primary Multi function display) but on balance keeping these elements discreet and actually having a reliable vhf gives me a new confidence. The failed unit was unreliable since late summer 2020 and I think had failed by the passage from Amsterdam to Stellendam giving problems crossing the Maas entrance to the Europort. Anyway, fitted and I hope forgotten for at least some years.