Hello helpful forum peeps,
Still not got to the bottom of this one. 
I have thoroughly scrutinised all connections, track gaps, and solder joints, and found them sound.
I have also tidied and refined the code as per suggestions.
I'm now convinced that the problem is somewhere in the midi input section, as I have discovered that the re-start does not occur when the midi cable is unplugged.
I based the midi input circuit on this
http://www.thebox.myzen.co.uk/Hardware/MIDI_Shield.html
however I found that this circuit periodically lost bits of information.
Through a little breadboard experimentation, I found that this was remedied by adding a 10k resistor between pins 7 and 5.
I figured that this was acting as a pull-down to logical 0 voltage level.
In any case, with or without the additional 10K, the reset occurs whenever all buttons are released, but only if a midi input is present.
Has anyone using this circuit experienced similar problems?
Any and all suggestions are gratefully recived.
Dan