Flexible duct - how taut?

[MouldyCandy]

We have been having issues with our bathroom extractor fan which I've discovered has likely been due to the TV aerial wire in the eaves lying over the duct and squashing it!
This is now fixed but how taut should the duct be? At the moment it's semi concertina-ed along the length from where it comes out the bathroom ceiling to where it vents out of the roof.
I'd like a steam free bathroom again!
TIA

Flexible duct is not very good because condensation, dust and fluff gather in the folds. If you are able to replace it with rigid duct that will be a great improvement and you can slope it slightly so any condensation runs outside, and may carry dust with it.

You can reduce condensation by wrapping the duct in insulation. The mineral wool loft insulation is suitable,

If you must use flexible, pull it straight. Apart from the short piece rising through the ceiling, slope it outwards.

It must not have any dips, because they will fill with stagnant ponds of condensation.

As you have a loft duct, you can use a powerful and quiet inline fan. Do you have one?

Edit

I see yours goes through the roof. You can buy insulated duct from ventilation suppliers. In your case, any condensation will run down into the fan.

Thanks PigletJohn. I replaced the original fan with an Envirovent one based on one of your previous recommendations. I'll get back up in the loft tomorrow and straighten out the duct.