We've had our water cistern in loft overflowing recently, the float valve seems to be working and today I found the water level considerably higher and it overflowing a lot. I believe the problem could be backflow from our kitchen mixer tap, this is the only mixer tap in the flat and has been recently fitted. All taps are mains fed and the loft in cistern just feeds the hot water tank. I tested earlier by running the hot in kitchen til pipe was hot then started mixing in cold and the pipe immediately went cold so I'm presuming this proves the backflow issue? What would be the best course of action? Can I install a check valve or shall I just change for a different mixer?