What does the red flashing s symbol on the dashboard of your vauxhall astra automatic 1996 mean?

I think it's telling you there is a possible problem with the auto-box.Corsa's are like this,so I'm guessing Astra is the same. The "S" should light up when you press the sport button on the gear selector,this causes the intermediate gears to hold on longer for accelleration or hillclimbing with a caravan on the back.You could try cycling the "S" button to see if it helps,though I think it cancels anyway with ign off. The "S" symbol flashes to alert you to a problem,it may require no more than a computer reset.The gearbox is checked for faults with a diagnostic kit,same as the engine.Hopefully,a short visit to a dealer will solve it for you.

We had the same problem with a 1995 Asta - I plugged the scanner in and cleared all the codes and the light no longer flashes, so your answer seems perfectly correct!