Mick - did you try the thing with the midi drivers yet? OPTIONS - MIDI DEVICES - de-select all drivers. If you've tried that and it doesn't make any difference, at least you can render to tracks and then merge the audio tracks as a work-around.

We had a similar problem to this , reported some time ago and eventually sorted it by a clean uninstall / re-install. Sometimes it's quicker than trying to work out what's causing the problem.

ROG.