It's a place called "Espresso Bar" outside the British Library on the main drag. It's managed by Peyton & Byrne.
Quality does vary from day to day and they put cinnamon in with the sugar (which I kind of like but does give it a "different" taste to what I consider the classic). The chocolate is OK, but it isn't Valor turned into warm cement with corn starch, which isn't necessarily a bad thing.