Sharpen up your codebreaker skills with these games from a new GCHQ puzzles book, seen first by the Telegraph. Think you've cracked them? More fiendish puzzles await...

The puzzles - scroll down for answers

General hint: Many of the puzzles may look more intractable than they really are. In a lot of cases the trick is to approach the puzzle with the right mindset – in other words to think like a GCHQ puzzle setter. So for example when looking at a set of words, don’t think of them as words, think of them as a set of letters. And when looking at a set of letters, think whether they could be converted into numbers. And when looking at a set of numbers, think whether they could be converted into letters.