I am replacing a solid-plate hob (5.5Kw) with a ceramic hob (6.4Kw) .
There
is a 30 amp fuse and cooker control unit. should this be OK for a straight
swap over?

No problem whatsoever, assuming the existing circuit is correctly designed
for its fuse. You should even get away with an oven on the same circuit,
although you'd need to check its rating. The total is allowed to go
substantially over the circuit rating in this case.

Christian.