Firstly, it is extremely rare for Corsa clutches to go even at very high mileages, unless it is an ex driving school car that the instructor(s) allowed excessive clutch slipping with during manouevres/hill starts or resting foot on pedal during normal driving. Secondly, due to the spring loading bias on the gear stick to avoid accidental selection of 1st instead of 3rd at high speed, it is only too easy to find oneself in 3rd,experiencing the symptons you describe (glancing at the gearstick won`t help as the gate is quite tight). Thirdly, the clutch cable could be sticking, both ends can be lubricated, or maladjusted.