I think everyone is missing something here. You've all given great advice about the fire risk of cutting metal with an angle grinder, but the tank is open topped and a small angle grinder placed inside the empty tank will shoot sparks into the tank, and not all the around the outside on to the loft floor.
It's also a lot easier to follow the cut when the tank is cut from inside out.
Not shooting you all down in flames here (pardon the pun), but there are safe ways to use tools if you think about the situation first. Casualty Departments all over the country are full of people who didn't think for five minutes before starting a job.
I'm sure all your advice is well heeded, but the sparks from the blade of an angle grinder don't have to fly everywhere if trapped inside the tank.