Luckily for most people, events in the C20th are still covered by living memory, even if it is somewhat hazy, so usually descendants can say roughly when someone has died, or might have died, because they know they were no longer around when they were born etc. So its helpful if you are in contact with any descendants of this line. Or is Robert likely to be in directories. I have tracked peoples' entries in directories to see when they disappeared, (of course they could have moved) or with men at this time through electoral rolls is also a possibility.
Regards
Valda