There's two types of flexible connectors one has a rubber O ring around the flexible pipe with a chamfered edge on the WC end and a compression nut and ring on the 15mm pipe end. The other has a chamfered edge with a rubber or fibre washer that sits in the end of the flexible pipe.
The type with the O ring is best for this type of job.
If the rubber washer is in place and the nut tightened, then it should be watertight.
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