With no Censuses available I turned to the 1939 Register and there struck gold, or thought I did, for I found a Frederick E Llewellyn (FEL) (my Frederick had no middle initial, or at least not given at birth), with the same birth date, married and living in Southend on Sea, in Essex.
Whilst still pondering whether they were one and the same I spotted a marriage annotation against his wife Phoebe Llewellyn, showing she became Mrs McDonald in October 1945. So, inevitably, given wartime, presumed FL had died, and likely had been a casualty of war?