CerveloMad

Fully agree with your thoughts Campag4life, on making certain that the front derailleur is set up correctly, on a mechanical groupset, but what I feel you have overlooked is that the bike I was working on is Di2 which is a whole new ball game. Believe me, I know how to set up a mechanical front derailleur correctly and also on Di2 having set up countless units over the past few years. The problem with the front derailleur on Di2 is that the distance to swing the chain from the big to small ring is electronically preset to match the chainset relating to the derailleur speed and in my case, an 11 speed derailleur, unsuccessfully trying to match to a 10 speed chainset. This swing distance is set at the factory to cater for the thinner chain and chain rings being closer together than on my particular 10 speed set. As far as I am aware, this preset cannot be altered and whereas on a mechanical set up you are able to adjust the derailleur cable tension to allow the chain to swing over further and drop accurately onto the small ring, Di2 has no such adjustment. Besides height adjustment, there are the 3 standard adjusting screws, Inboard stop, Outboard stop and Angle and all were set perfectly. The shifting was always attempted at high cadence on a bike stand so there was no languishing either during gear changes as you had suggested.
The proof being in the eating, after fitting the new, slimmer 11 speed chainset, the chain dropped from the big to small ring perfectly without any other adjustment to the derailleur. To reiterate my initial post, not all 10 speed chain sets have the same ring spacing and when dealing with Di2 systems, you may get away with some 10 speed chain sets if the ring spacing is small enough, but as in my case, just 1mm can make all the difference.