Diagrams were in the Chiltern post in the "Frequently Requested Diagrams" thread, but appear to have gone walkies last time it was updated.
Mondays to Fridays (until 16th May) the 0645 and 1518 from Marylebone, and 0914 and 1735 from Stratford-upon-Avon, are diagrammed for 172s; on Saturdays they form the 1309 from Marylebone and 1537 from Stratford-upon-Avon; and on Sundays they form the 0930 and 1509 from Marylebone, and 1138 and 1738 from Stratford-upon-Avon.