Have finally had a result (with the help of a professional researcher)
It transpires that my Grandfather was NOT born Robert Valentine Walker in 1898, but Valentine Henry Down in 1897, eldest of Richard Gardner Downs (13+) Children. I now have his Birth Certificate and 1901 Census entry where they were already using an assumed name of 'BRITTON'. Unfortunately, no sighting of him, or Richard Gardner Down on 1911 Census, despite extensive research. Also no Birth or Census info for his half Sister, Winifred Ethel Clements so possibly not her Birth name.
Still, happy with what I've found out😀