Try putting £20 of super in it when the light comes on and see if it runs better. Mine ran loads better on super unleaded due to the fuel pump on it although after reading the post above could of been the pressure regulator but it was down to the fuel anyway. Easy way to test to see if it is fuel related, if it makes no difference you've not lost anything then look into the spark side. Its possible it could be the coil or leads breaking down under load but it might also of had cheap or the wrong plugs put in and not the ones for the ST but if it was me I'd try super first as you have to put petrol in anyway.