Thank you for the replies. I think, after agonised pondering, that the part I tripped up on was the transistor. Now after staring at the schematic until my head ached, I think I get it.
Correct me if I'm wrong. Between the input and the op amp's output there is a voltage of Vin + VBE, but as far as the current is concerned, I only care about the voltage difference between the input and the op amp inverting input, which is just Vin (since the inverting input is held at 0V), so the current is set at Vin/R1. I think I was kind of ignoring the transistor, doing the calculation as if it weren't there. I couldn't imagine what a fixed current would do when it came out of the emitter and encountered -VBE volts. To be honest I still can't: flow, I suppose.