I asked the historian Richard Baxell if he had any information about Otto Estensen and he replied with this:

This is a brief summary of what I have on Otto:
Born Thormely on Tees
Jewish
Address: Halcroft, Redear Rd, Ormesley, Middlesborough
Occupation: ABS aboard Ship
Member of National Union of Seamen, Middlesbrough branch since 1925
Joined CP 1933
Arrived Spain 18 May 1937
IB ID no. 1103
Served with anti-tanks and British battalion
Repatriated December 1938

Thank you Richard.

Richard is the author of
British Volunteers in the Spanish Civil War: The British Battalion in the International Brigades 1936-1939, which is an excellent authoritative account of the British and Irish contribution to the SCW. http://www.amazon.co.uk/British-Volunteers-Spanish-Civil-International/dp/0955419905