While keeping an open mind on this one, it does appear that the explanation from the Johnson camp suggests that this might be a damp squib:
We had no reason to believe the donation came from anyone other than (Mr Siddiqui)...We checked he was a member of the Labour Party and was on the Electoral Register and we registered the donation with the Labour Party, the Register of Members' Interests and the Electoral Commission.
Do we have to go through every donation to every political party over the last twenty years one by one in the media spotlight? It's getting a bit tedious.
Hat-tip to Peter Black.
There's still Cruddas, Benn and Blears left of those who contested the Labour Deputy Leadership but from whom, to my knowledge, there hasn't yet been a donations row about. So there's plenty of fodder left.