Best Way to Seal a Fan Vent

[slangivar]

This picture shows the fan vent coming through my bathroom ceiling and heading up through the roof:

Where that initial white flexible pipe goes into the black pipe there is no sealing which I assume is far from ideal. It means that some of the bathroom ventilation ends up in the loft. I could simply wrap some tape round the join but I assume there must be a better way to join the pipes and wonder what it is.

How would you go about sealing this joint?

 

[Hot&Cold]

Have you considered removing the black flexi pipe and try to extend the white concertina pipe up to the straight black pipe adjacent to the yellow label ? If it fits then secure it with a large cable tie or duct tape.

 

[slangivar]

Thanks. That's not a bad option although the concertina pipe is the same diameter as the top pipe but I think I can make it fit. Unfortunately it's too short. Before I buy a longer length I thought I would check the other 2 fans in the loft. One of them is fine and the joint is nicely taped while the other one simply has the black pipe sitting over the fan outlet with nothing sealing it. How airtight should a fan duct be? Should I redo this one at the same time?