"Andrew P Smith" wrote in message
...

My understanding is that Chiltern trains must enter the LU controlled
sections in a specified 'time slot' and if they miss this slot then they
follow on behind a Met train if that train is ready to enter the
section.

And right on cue ....

Tonight (Monday) the 19:15 Marylebone to Aylesbury (first stop Great
Missenden) departed approx 30 secs late from Marylebone (due to the last
minute substitution of a Class 168 from an adjacent platform, as the
country-end 165 of a pair was defective, trapping in the set to form the
service). Arrived H-on-H virtually on time, crawled through the platform and
then staggered all the way to Amersham. Why? Because the buggers in Harrow
box had let out an all-stations Amersham stopper in front of it! Again.