I went back at lunchtime a few days' later to check it out. Inside, it's cosy and, as I mentioned, rather diminutive. There is a bench outside on the pavement and a couple of spaces to sit in the window, but otherwise, it's elbow-room only (and not much of that). Nonetheless, on a sunny Friday, there was a constant stream of customers, including many regulars, and the friendly baristas kept things moving very well. The sleek white espresso machine and monochrome menu and neon sign add some edge to the rustic exposed-brick walls and coffee-sack-lined window seat.
They roast their coffee locally and there were five single-origin espressos available when I visited. There aren't any hand-brewed filter coffees on the menu but you can try several 'signature drinks', including the Great White and the Great Long Black. There is also an assortment of toasties and various sweet treats on offer.
I had a piccolo (£2) made with a blend of a couple of Central/South American coffees and it was expertly brewed. The coffee tasted smooth and rich, and the latte art was very good. As most of their customers take their coffees away, be warned that they serve the coffee in disposable cups, so you may wish to consider bringing your own reusable cup — or if you ask nicely, like I did, they may be able to rustle up a glass for you.
This neighbourhood, on the boundary between Bloomsbury and Clerkenwell, has several other excellent coffee shops — most notably Prufrock and Catalyst — but if you're in the market for a quick but very good espresso-based drink, don't overlook Hopper Coffee.