Some work that GCHQ carries our is "necessarily secret," Sir Iain Lobban said.
"I don't think secret means unaccountable in any sense."
"I don't think secret means sinister," he added.
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Temperatures are predicted to reach a high of 23C.
Hunt accused companies of "turning a blind eye" to a generation of children "exposed to harmful emotional side effects" of social media.
Top temperature 24C/75F.