The only problem heating the top of the tank is when you draw off hot water it will be replaced by cold water which will come in the bottom of the cylinder and start to cool the top .
I used to work for Kingspan and they didnt recommend it because the cylinder was obviously at diff temps ...
realistically you cant , you will only be flushing the pipe , last one I did , all the rads came off and were pressure washed in the customers back garden , I advised her it would never be right until it was converted to a two pipe , her rads get hot but I have still had blockage probs .