What does a flashing 'F' light mean on a Corsa automatic?

It means the fuel is low.

No it doesn't! It means that there a Fault with the automatic transmission.

As it says in the manual, when this F appears it indicates a serious problem with the automatic transmission electronics and you should get it seen to asap.

THIS means there is a fault and in 9 out of 10 cases it is a faulty crank shaft sensor as i have just replaced one the cost is about £9 and a easy fix it is located next to the oil sump