Yes Rosie, I think I was thinking of a slightly larger terrace in my suggested layout of the rooms in this house.

I did look at the back of the houses across the road from 5 Cowper St (because you can't see the back of 5 Cowper St from Google Streetview). From looking at the maps we know that the houses opposite 5 Cowper were built slightly earlier than the row where No 5 is situated. I believe the houses are the same though - probably built to the same plan. Looking down the back alley you can see the backs of the houses, and what is most likely a bathroom window at the back. So I assume that was originally bathroom when the houses were built.
I would have expected to see sash windows on houses of that era however it looks like
all of the houses in Cowper and surrounding streets have the 60s looking windows with the narrow bit at the top and the large fixed pane at the bottom. The windows at 5 Laurel St caught my eye - they look authentic and would explain the fact that there are
no 50/50 split style sash windows (even replacement ones) on any houses in the vicinity).
Regarding the doors, I think there is an original on the house round the corner at 59 Mills Hill Road. This is the only one I have found (so far).

Rubymelia, if you can imagine those features on your house at No 5, I think that is how it would have looked when your relatives lived there.
