I wonder if we are waiting for some 350s to reach the end of their lease ?.
I wonder if they already have. Last Saturday, Avanti's 15.21 New Street - Euston was cancelled (usual reason) and there was ticket acceptance on the LNw 15.36. departure to Euston, Unfortunately, the latter was only a 4 car and it was not really possible to board it at 15.25. The next LNw was the 16.06, and this was also 4 car (not so much of a problem - it was pined* at Northampton!).
So unless there's been a rapid decline in maintenance standards at Kings Heath the shortage might be explained by 350/2s having gone off lease. The short formation was unexplained (i.e. not the usual 'more trains than usual needing maintenance') so perhaps it's not King's Heath's fault.
Nothing to do with LNw but the next Avanti departure to Euston (15.47) was an 805 instead of an 11 car Pendo...
*Pine - terminated before destination.