I don't think you are right about Northampton on a Sunday? The first 'down' services are 09h06 to Birmingham/09h27 to Crewe, which is not that much different to Wellingborough.
Sorry for the delay, it wasn't because I was trying to weasel out of it!
It's all a bit confusing to me as I can't find my original post (which may have mistakenly given 06.42 as the time of the first Northampton - New Street train on Sundays) or the one it was a reply to; but the intention was to point out that the first service
from New Street
to Northampton is around 06.42 on Sundays, and a similarly early departure from Euston, the point being that these were well before the arrival of the first train
from Northampton
to each.
The thrust of the proposition was a response to the suggestion that, because the maintenance depot is at a particular location, Derby, then the first services will be outward from there with the inevitable result that the return services (from outer termini) can't run until the first outward service has reached there. My proposition was that the first trains from New St and Euston leave before the first trains arrive from Northampton (the location of the maintenance depot for LNw, like Derby is for EMR), by stabling trains at the outer ends of the network rather than all returning to depot.
But possibly you're referring to another line of thought. It's only me that's confused , I know...