2004 Vauxhall Astra engine management light

Wondered if anyone could help. My astra lost power and the engine management light started flashing which has now stayed on permanantly. After switching off and restarting the engine the car runs fine again. I took it for a code read which indicated an empty fuel tank. The tank was half full at the time and hadn't run particularly low when I last refuelled a few days before. The fault was deleted but the engine management light was back on a few hpurs later. I took it back and the code read was the same (but also with a code of P1300 which isn't listed anywhere.) I normally use Shell but used supermarket petrol and the last refill so the likely problem was a dodgy batch that possibly had some water in it. I've been advised to put some dry fuel in the tank when I next refuel to sort the problem but if this is the case is there a way I can switch the engine management light off without having to go back and pay a garage to do it.