Thanks for the replies so far. Excellent.
I have searched the net and there are sources that indicate that no one knows whose decision it was to not advise the crews. Or indeed if there was ever a decision - to be 'not advised'.
Hindsight is wonderful but my feeling is that given the immensity of the entirely justified attacks on Nazi Europe the loss in payload compared to the savings in aircrew and airframes should have dictated some sort of ventral defense.
It is humbling to read of the losses of crews but especialy saddening to speculate that something could have been done to minimise losses.
Another thought prompted by the responses is 'How widespread were the 'unofficial' modifications.....?' I'd love to hear if a BC veteran had any comment.
Cheers.