Dr Alf is correct. Jeremy Corbyn and his followers are very much in the Foreign Office Arabist mode and would probably like to see Israel be eliminated as a Jewish state and a two-state solution for Palestine imposed. They are for “multiculturalism”, which means that they are keen to ingratiate themselves with groups of migrants that they think might vote for them.
The cold calculation that they have made and still make is that the Muslim Community and other groups are more likely to vote for them whilst the Jewish community is more likely not to. If that then means buying into anti-Semitism then so be it. Jeremy Corbyn himself can claim to be moderate, reasonable and polite and can say “Nothing to do with me guv” when it comes to the antics of his followers. The extreme-left worldwide has increasingly come under the influence of Muslim political groups.
Meanwhile, David Cameron’s attack on Corbyn was possibly influenced to some degree by sympathy, given his own partially Jewish origins.