I've been doing a bit more research and also spoken to a friend who is more tech savy than I am.
The research and the friend have suggested the same answer and that is the two "hidden" partitions are recovery or diagnostic partitions for the PC manufacturer. Dell in my case. Taking it further given that the Acronis back up of drive "C" will include the OS there is no overriding need to back up the "hidden" partitions. In any event they will not change so if they are to be backed up it only needs to be done once.
I have therefore done a whole disc back up of all three partitions and will do incremental back ups of "C" only with the occasional consolidating whole disc back up when I have too many incremental back ups if that makes sense.