Quick update, after riding with the Tiagra shifter for a few days I am no longer so sure that the lever was the sole problem. While shifting has improved significantly, it's still not quite perfect, and shifting into big ring still difficult when under load, but OK whilst descending. This leads me to believe the FD may have been at fault all along. Perhaps went on to damage the lever whilst I was tinkering in the garage. Finding that loose spring cannot bode well for the long term reliability of the shifter anyway, so it needed replacing.
Well, I've picked up a new 5700 FD and a used but mint ST-5700 left hand lever on
ebay, I'll fit the FD first and determine whether it confirms my suspicions about the FD. Then to restore some coherence and order to the styling, the 5700 lever can go back on... Sadly this one was silver, the original black, so I'll still be slightly mismatched but not quite as bad as that hideous ST-4600 model. At £21 I don't think I'll bemoan an odd colour scheme... Then again, it's an excuse to attempt a custom paint job on the levers. How's about pearlescent shifter levers?
