Sorry, am I being thick here? Why would there be an issue with a stopping service following an inter-city one?
Presumably, because a train which leaves at xx.12:00 (the XC fast train) will probably not pass the end of the platform until after xx.12:10 and will probably not clear the block after the end of the platform within 30 seconds, meaning that the starter signal for the local train won't clear until after xx.12:40, meaning the dispatch staff can't give the station work complete signal to the guard and have all the doors closed and the signal not showing red ready to give the driver the RA at xx.13:00.
In the old timetable Cross-city trains departed at xx.04, xx.14, etc. giving two minutes between the XCs and the stopper, and it was possible for a Cross-city to leave bang on time. Given the station work complete signal needs to be given
at least 20 seconds beforehand for the train to depart exactly on time, I doubt it can be done (but would love to be proven wrong!)
I don't have a copy of the working timetable, but it may very well show the scheduled departure time one or two minutes after the public.