simon mitchelmore wrote

Condensation is forming inside the 100mm pipe above
the bathroom vent fan which passes through the cold
loft (about 10 feet) to a grill at the apex; the water then
drips down through the fan when its off. This is in a flat
hence the bathroom has no outside walls and also I can't
re-route the 100mm pipe horizontally out of the roof.

This is as was fitted from new (1980's) but the first
winter for us hence this must happen every day in
cold weather when we take a shower.

Any suggestions to stop the condensation forming?

Insulating the pipe might reduce the condensation but not eliminate it.

I've even considered fitting some sort of gravity condensate trap which
would have to then have a drain pipe leading out through the eaves.

Yeah, thats the only really viable approach.

The vent fan comes on when the lights come
on so every visit gets the vent fan being used.

It wouldnt be hard to fix that.