Bill: I think best play depends upon the ko threat situation and the rest of the board. But, since the kos are so big, I am going to assume that neither player has a ko threat, and also that the temperature of the board is not too high, so that there will be no ko exchange.
takes ko. Later, - W a is sente.
White makes ko with . If instead, Black can respond with and kill.
With no ko threats, Black has to protect with and . escapes.
Grauniad: Doesn't this allow White to escape without a ko? If at 3, at a kills 4 Black stones and lives.