The problem appears to be that the Victron multiplus is fairly fussy in the quality of the incoming 230v feed ( frequency stability, wave form, voltage stability, noise) that our current generator does not meet, therefore it does not switch across to charge/ inverter bypass mode. If any one can supply details of a generator that does achieve this, including any, if required, setting changes to the Victron, then this, at this stage would be my preferred option rather then installing a dedicated battery charger.
Cheers Don