Hi Jacqui
Did you look at the JewishGen Unified search of surnames Cywan,Syvan,Sylvan etc? You may never know why they changed their names but don't forget the 1939-1945 WW2 and the earlier 1914-1918 WW1 where many Jews changed their surname on arrival or post arrival in the UK and elsewhere. Similarly, the surname changes when Jews first arrived in the UK post 1658 Re-Settlement.

Your father is the Daniel Barnett Sylvan Hebrew Beryl Dovid B/Y Yaacov died Feb 21 2001 South Africa- is that right?

The majority of the Cywan surname are from Lomza Gubernia Poland or Warsaw- the spelling/pronunciation would be very different in Polish