I seem to have figured it out, but I don't quite understand it.
In my amavis/conf.d/20_debian_defaults, on the debian server I have this line
$final_virus_destiny = D_DISCARD;
according to this page
that means it should dump the mail and tell no-one, however it dumps the mail and tells the sender
On the ubuntu server, D_Discard behaves as it should (IE, telling no-one) so I've changed that to D_BOUNCE and now the sender is getting notifications.
Anyway, does anyone have an idea about how to send notifications to the reicipient?