Thanks for that, I hadn't realised (or forgotten) that.
One might jump to the conclusion that it's a 3 car being tested (from the livery) and, in further steps of 2 + 2 equalling at least 5, that these will be entering service before the 5 car ones. Extending speculation to possibly beyond the limit of elasticity this might mean the 323s (including the ones to be transferred from Northern) might be terminated before the 350/2s. If so, strange that the 323s have been refurbished recently, but the 350/s remain tatty. I am basing all this on the assumption (but have no knowledge) that acceptance of 3-car units can't simply be carried over to 5 cars without actual testing of the latter.
But I still wonder how siding capacity, driver and other resources will be available for acceptance of the London Northwestern fleet; presumably, it will have to be an overnight replacement of the 350/2s - 730s not (I think) being able to mult with 350s.
But I'm sure that there's clever people in WMT who have the solutions already planned, I'm just not bright enough to work it out