Oh dear. Looks like the 1897 census would have been perfect, but as I read it copies destroyed except for Archangel and Tobolsk.
"Each enumeration form was copied twice, with the three copies filed in the county archives, the governorate archives, and the Central Statistical Bureau in St. Petersburg. The copies in St. Petersburg were destroyed after they had been tabulated.[4][5] Most of the copies stored at the local and regional level have also been destroyed; however, the complete census for the Arkhangelsk and Tobolsk governorates has been preserved, and the census for portions of several other governorates is also extant.[6][7]"Now, my Russian geography is less than poor, so I cannot work out if St Petersberg is in Arkhangelsk.
But I guess this means there is no online access to this census.

And visiting is not an option.
So I may never know why this rellie was in St Petersburg, or for how long.
That's Family History for you!!!
Thanks for all your help.