thanks for the reply, Paul.
I downloaded the MIDI library from Github, details can be found here...
http://playground.arduino.cc/Main/MIDILibrary#.UzrJrfldWC0
I'm not convinced sendRealTime is the right method to send the clock either, it's just what I found in the library for sending the clock.
I've seen ways to do it at a lower level (without the library), but I don't want to mix methods for sending MIDI, and the library seems useful for anything more advanced than sending note on/off, etc.