Selgas
You have checked the thermostats I suppose cause of one does not operate when it should then you will not have power to the lower element. By changing the red and black wires around will not do anything except cause the switching to take place between the neutral instead of the phase side of the circuit.
Check you have good earth connections as well and if everything else fails then call in a Sparkie cause your problem may be at you meter board and the relay device (if fitted) on it.
Hope this helps some. 
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians
