We had a similar problem - pools of condensation, mainly in the length of flexible duct that was lying flat on the loft insulation, eventually overflowing and dribbling back down the vent.
My 'insulating' was done with many layers of plant fleece and duct tape - supposedly temporary until I replaced the corrugated duct with something smooth walled and properly fitted, but that was five winters ago and I still haven't done it.
I also removed some of the excess flexible duct, giving a better flow rate and a smaller area for heat transfer and subsequent condensation - there was around 3m of duct in place when only 2m was actually needed.
Increasing the run time of the fan helps: there is always going to be some condensation until the inside of the duct warms up above the dew point - this condensation can only be removed by running back down the duct or evaporating into air that has a relatively low humidity.
Now my problem is that the current bodge seems to work well enough, so there's no real motivation to go back into the loft to do it properly...