"geraldthehamster" wrote in message
...
Thanks to Messrs miaow, Wallop and Hansen, and to everyone else who
replied. I'll go with the separate vent and chimbley, which will in
any case be less fiddly to contrive.

Concerning condensation within a vertical vent that passes through a
cold roof space, should I pack insulation round it?

Regards
Richard

Insulation won't really help. You do have to allow a water trap at the
bottom of a vertical stack to catch any condensate that trickles down the
vent. Something that you can clean out and dry off when it needs it. We
used to put a cap end on the bottom of a 4'' pipe stack being used as a
vent. It lets you unscrew the bottom off and dry it.