my general concern is that hot water is being passed into the tank from an overheating appliance, either the boiler or a cylinder if you still have one( in the news last week re the little girl) and the item needs to be checked out by someone competent to do this, as it sounds like a fault occuring.
1( is the water in the tank warm or hot? the fresh water coming in will be very cold, as it is from underground
2) are the rads cold all over or cold at the top i.e. trapped air or cold at the bottom i.e. trapped sludge
3) which water level do you wish to reset ?
before answering i think you need assistance for the hot water in the tank problem and should make arrangements for this to be assessed