Trouble in Loft

[monday]

This house was built 1997 and we have lived here since built. Noticed recently a small dampstain on bathroom ceiling and checked the loft. The soil pipe vent is boxed in a corner of the bathroom, rises up into the loft space in (roughly) 150mm uPVC, takes a 90 deg bend to run horizontally directly above the stain, then connects to a corrugated flexible pipe to go vertically up to a roof vent fitting. The connection between the uPVC and the corrugated pipe was pretty poor, held together with what I would describe as “builders parcel tape”. At first I thought this joint was leaking rain, driven in through the roof fitting, so reinforced it with duct tape. But water accumulates on the bottom of the horizontal uPVC (just enough to feel with fingers) but paper towel laid on top of the pipe remains bone dry.
Could this simply be condensation? Warm air in the vent, cold air in the loft? The roof is insulated to the standard of the day, about 150mm mineral wool.
But why have I only just noticed it after 23 years! Has it taken this long to get through the plasterboard?
Comments appreciated.

 

[foxhole]

Lag the pipe , should stop condensation.

 

[garyo]

But why have I only just noticed it after 23 years!

Covid? More people in the house more often =more vapor?

 

[monday]

Foxhole: Thanks for lagging suggestion, I should have thought of that. Found that Wickes sell 7m x 100mm wide lagging which should do the trick.

garyo: no, no change in occupancy. But did occur to me we had gutters and soffits replaced last year, still ventilated though but may have changed the atmosphere enough?
Will lag and see what happens.

thanks all.