I'm looking for a novel I owned and read in the 1980s. Here's the little I can remember about it:
The main character is a teenage boy who has studied the Sherlock Holmes books and tries to follow the great detective's methods. (For example, I think he described one puzzle as a “three-gobstopper problem”.)
It's written in English and set in London, probably a suburb, and is contemporary and low-key. He and his friends have a large room/clubhouse on the upper floor of a warehouse or similar building. They get on the trail of some baddies who have a (blue?) van, and a hut or other hideout somewhere in the Essex marshes; the MC later identifies splashes of Essex mud on the van from the colour. (I don't remember what the baddies had done, but it might have involved a kidnapping.) Near the end, he and his friends get holed up in their clubhouse, watching the sun go down, knowing that the baddies are out there, closing in, and will attack once it gets dark…
This was a UK paperback, maybe a Puffin, probably fairly small, and I think it had a plain light-blue cover with a simple monochrome print or line drawing on the front (unless I'm confusing it with the Puffin edition of The Hound Of The Baskervilles). [EDIT: A Piccolo; and yes, I was getting the cover confused: it's mostly black-on-white]
It didn't seem like part of a series. [EDIT: Actually the first in a series, though not billed as such.] I think it was fairly new at the time, so probably published in the 1970s or early-to-mid 1980s. [EDIT: 1980] I may have bought it new from a book club/catalogue such as the Puffin Club (though I've checked all I could find online, and nothing rings a bell).
That's all I can remember, I'm afraid — except that it appealed to teenage me!