In case anyone's interested, the hesitation seems to have been due to the crank sensor. I checked the air gap using a feeler gauge, it was in spec. Took it for a 100 mile ride and the problem had gone away. Done 300 miles since, no problems.
My fanciful theory is that I dislodged something (magnetic? metal filings?) from the sensor when shoving the feeler gauge blades in.
So having repaired the PC wiring, the PC and DP cans are back on and I am a happy bunny.
This is what it was doing before:
http://www.youtube.com/watch?v=HdDC6djC9hU Again, thanks to Neil at Cornerspeed for his excellent advice.
Cheers,
Martin.