Auto Transmission problem

[Toodeep]

The automatic transmission on a 1.6 Privilege keeps going into what is presumably a 'get you home' mode where it stays in what seems to be second gear accompanied by the message "Check Gearbox". It has done this once or twice in town but now that my goddaughter has a weekly commute between London and Farnham, Hants, it does it almost every time. Pulling off the A3 and switching off tends to rectify it but I wondered if anyone else had suffered anything similar. There doesn't seem to be anywhere to top up with fluid and the transmission seems fine otherwise.