Just out of interest, how many Kw are all your appliances added together?
You say you have 2 x boilers and 1 x cooker?
It could be that your meter is also undersized.
Never heard of them, but to wire your proposed new one you would need to do the following...
Terminal 1 (existing stat) to 4 (new stat)
Terminal 4 (existing stat) to 3 (new stat)
Terminal 2 (existing stat) to 1 (new stat)
Hope this helps.