I think that what you have done is to connect the waste pipe to the trap of the sink. There is a smaller take off pipe that you have pushed the waste hose onto.
If that is correct, you have not sawn off the end of the white plastic take off pipe.
When the waste pipe is fitted, the take off spigot is sealed so that if you don't need it, the waste water wont pour out of it.
When you need to connect a waste hose, you saw off the very end piece, so the waste water can enter the trap.
Push the hose on tightly, after you've cut the end of, and hold it in place with a jubilee clip. You can get one from B and Q.
You shouldn't then have any more trouble.
If you need more help, please just ask. In the meantime,
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