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Leaking tap

I have a toilet that is fed by mains and tank water. It has been running off mains water but I'm wanting to switch over to the tank option however the tap that controls the mains water going into the toilet has a leak and needs fixing as when I try to turn it off and just use the tank water the pressure seems to cause the mains water tap to leak. I've attached a photo to see both taps. Thx