Just an update in case anyone else comes across this thread with a similar issue. Finally got round to getting the fuel tank and air box off this weekend. Synced and cleaned the throttle bodies. Issue resolved immediately - so after 8 months or so and having been in checked by two seperate mechanics (one holding it for over 2 weeks and unable to fix) it was as simple as that.
Pretty unbelieveable and slightly peeved as at the second garage, I said on the phone 'have you synced and cleaned the throttle bodies' and was told this would not be the issue and if it was it would have been picked up by the diagnostics (no idea what diagnostic there is outside actually stripping the bike down and attaching a manometer to the throttle body - but hey I am no expert.)