The ticket you are asking about carries the following restrictions:
Restriction Code
SN
Applicable days
All days
Outward Travel
Not valid on trains timed to depart:
London Terminals before 11:00 and after 13:40 until 20:15, Mondays to Fridays or before 10:30 and after 16:30 until 18:30, Saturdays and Sundays;
Watford Junction before 11:30 and after 14:40 until 20:37, Mondays to Fridays or before 10:35 and after 16:35 until 18:35, Saturdays and Sundays;
Milton Keynes Central before 12:10 and after 14:15 until 21:10, Mondays to Fridays or before 10:45 and after 16:45 until 18:45, Saturdays and Sundays.
Return Travel
Not valid on trains timed to arrive:
London Terminals before 13:00 and after 15:20 until 20:30, Mondays to Fridays or before 12:08 and after 17:50 until 20:05, Saturdays and Sundays;
Watford Junction before 13:10 and after 15:20 until 20:10, Mondays to Fridays or before 11:53 and after 17:35 until 19:50, Saturdays and Sundays;
Milton Keynes Central before 12:15 and after 14:20 until 20:15, Mondays to Fridays or before 11:42 and after 17:25 until 19:40, Saturdays and Sundays;
Rugby before 12:20 and after 15:25 until 20:20, Mondays to Fridays or before 11:34 and after 17:00 until 19:30, Saturdays and Sundays.