So, I dropped the car off to the workshop this morning and got a phone call a few hours later. After pulling the gearbox apart the guys found out that a main bolt inside had come undone, dropped and broke 5th gear into a few pieces and then lodged in between the selector forks. They've never seen this happen before and as the gearbox has never been opened since leaving the factory,would indicate a manufacturing fault.....too late now. A new 5th gear set and I'm also getting a new clutch whilst the gearbox is out, all up ~$2200. I'm just glad it's not a new gearbox job