At a track day at Woodbridge a couple of weeks ago the car jumped out of gear a couple of times (3 or 4 in total in about 6 hours of driving). It's never done that before, do I have gearbox problems brewing or could it be something simple like clutch cable needs adjusting.

Would not have thought it would be clutch cable adjustment. Could it be in the that due to driving quickly you did not engage the gear fully and it jumped back out.