It's funny what I find as I'm browsing the library shelves. I didn't know, until I picked it up, that Robert Westall had written a sequel (of sorts) to The Machine Gunners. I say a sequel of sorts because Fathom Five doesn't really allude to the events of The Machine Gunners. Chas McGill is now 16 and looking forward to the long summer vacation from school. One day Chas and his friend Cem find a strange object in the river - it's a large enamel bowl with a cardboard box glued inside the bowl. In the box is an empty cigarette packet, a gold watch with the hands tied together with yellow wires, which lead to a battery and a grey cylinder. Cem immediately assumes it's a bomb and they race away up the beach. However, Chas returns and investigates. Eventually he works out that the cylinder is actually a Morse Code tapper (the device that's used to transmit Morse Code) and then Chas realises that the equipment belongs to a spy. He talks Cem, and his other friends Audrey (who's now a reporter and apparently still "almost as a good as a boy") and Sheila (whom Chas fancies) into helping him to find the spy who's operating in Garmouth. Along the way he nearly gets the four of them killed in two separate dangerous situations, gets into trouble with the police, and inadvertently gets someone killed. But he does find the spy - but even that turns out to be a more complicated situation than he originally imagined.
This is an interesting book and became more and more compelling the further I read. You don't have to have read The Machine Gunners to read and enjoy Fathom Five (probably because in the original hardback edition, Chas was an entirely different character). Westall wrote a third Chas McGill book: The Haunting of Chas McGill, which I'll be picking up from the library on my next visit.