I have spent more than too much time on this but have managed to find a way around the problem that I have with sending Calendar Reminders. The way that I have found is in no way elegant, but it does work for me.
The process that I am currently using is to create an *.ics file within Filemaker and then send this as a mail attachement to the required recipient. Once the recipient opens the attachment a calendar event (in MS Outlook) is created, ready for the user to save.
There are some problems that I have come across, the *.ics file does not open correctly with iCAL on OS X, but this is a small hurdle that I hope to get past in a bit more time. At current, my system will only be Windows supported, until I can invest the time to make is Mac compatible.
I'm hoping in time that I can get this to actually send as a reminder rather than an attachment, but this will require a lot more research.