My usual haunt is the Thameslink route on the southern portion of the MML, so I'm used to fairly spritely EMUs with a maximum 2 hour journey time end to end. The big difference is most routes are 4 track with a fast and slow in each direction, so it's fascinating to see such a variety of services using so many routes with only 2 tracks.
Thameslink typically operates a fleeting timetable with an express, followed by a semi-fast, followed by an all-stations and then the timetable repeats at either 15 or 30 minute intervals.
I can imagine a similar timetable of express, semi-fast and all stations every 30 minutes could work for the Piccadilly to Airport route, however the difficulty appears to be the unreliable time-keeping of some express services with many originating as far away as Middlesborough, Scarborough, Hull, Cleethorpes and Wales. It's hardly surprising these services often formed of an overcrowded 3-car DMU laden with luggage are delayed enroute, given they are competing for track space on congested 2 track routes. While Thameslink does need to accurately deliver trains to the twin-track core section, the routes either side of the river allow trains to overtake or skip stop to achieve an on-time delivery to the Core. To be honest, a train arrival within 15 minutes for a service that started it's journey 3+ hours ago is pretty good going. The only difficulty is 15 minutes is enough to put the TPE behind the local stopper.
I witnessed a TPE 185 from Cleethorpes arriving 40 minutes late into Piccadilly. They brought the service back to schedule for the return departure from Piccadilly by omitting the run to/from the airport. Not great for the many customers with suitcases going to the airport or those waiting at intermediate stations, but it was a wise decision to recover the timetable.
Is the intention for TPE to keep enough 185s to ensure all TPE services are 6-car formations ? They could certainly do with longer formations to cope with the increasing passenger numbers.