Nigel,
Try sending controller # 30
Values upper = 00, lower = 00 for off
Values upper = 16, lowe = 16 for slow
Values upper = 127, lower = 127 for fast
(in other words send the 00, 16 or 127)

on the midi channel you are sending notes to (playing on)

Not sure if it is correct, but worth a try.

Lee
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Lee S.