I had a leak in my toilet and that was fixed and then straight away the hot water stopped coming out of my mixer tap in the kitchen, the cold water is fine.

Is this a single handle faucet of a two handle faucet. Either way it is porbably a stopped up line goming to the faucet or it is something in the faucet. I suggest you turn the water off and take the faucet apart and flush out the body of the faucet.
If this does not work take the pipe loose that goes up to the hot side of the faucet and run some water into a bucket from this pipe and see if you get any water out of this pipe. These test should tell you somethng.