When I manually open an .ics file that I have generated (by double clicking on it from the Desktop), it all seems to work, with the exception of the reminder/alarm. For some reason, this always opens with the default state where Reminder is set to none.
Could you let me know if this is something to do with my code, or something that Outlook (and possibly other software) does?
BEGIN:VCALENDAR
PRODID:-//Company name//Product name//EN
VERSION:2.0
BEGIN:VTIMEZONE
TZID:Europe/London
BEGIN:STANDARD
TZOFFSETFROM:+0100
TZOFFSETTO:-0000
DTSTART:16010101T020000
RRULE:FREQ=YEARLY;WKST=MO;INTERVAL=1;BYMONTH=10;BYDAY=-1SU
END:STANDARD
BEGIN:DAYLIGHT
TZOFFSETFROM:-0000
TZOFFSETTO:+0100
DTSTART:16010101T010000
RRULE:FREQ=YEARLY;WKST=MO;INTERVAL=1;BYMONTH=3;BYDAY=-1SU
END:DAYLIGHT
END:VTIMEZONE
BEGIN:VEVENT
CREATED:20161008T144102Z
UID:4989C88E4BD54DFF82864D58CBFF12A6AD68ACD9BF3344AA84FEC7683C4DA
DTSTAMP:20160925T093000Z
DTSTART;TZID=Europe/London:20160925T093000
DTEND;TZID=Europe/London:20160925T210000
SUMMARY:Here is a summary.
DESCRIPTION:Here is a description.
TRANSP:OPAQUE
BEGIN:VALARM
TRIGGER:-PT1440M
ACTION:DISPLAY
DESCRIPTION:Reminder
END:VALARM
END:VEVENT
END:VCALENDAR
Dont know when you last tried the above event but the DTSTART/DTEND are both in the past. As a consequence, I suspect that the client is just ignoring an alarm that it can no longer trigger. + You might want to express the TRIGGER in terms of hours (-PT24H).