UPDATE: I have now successfully found a way to make the chrono work for me. The start/stop button doesn't just need a firm press, but a firm, quick one. Takes a bit of practice but now works every time. As it still fails when not using this technique, I still consider it a fault - when clicked, the lever should engage properly regardless of the nature of how it was pressed - but it's a fault I can live with until a service. If I get round to taking the movement out, I'll try to reseat the chrono hand as I'm getting the classic 'does not reset to 0' problem. I figure this one is due to a loose hand as where it ends up seems completely random each time (anywhere within +/-1.5 secs) and it easily falls out of alignment with a gentle knock on the side of the watch case.