I think you're right about it being a soil pipe dave. Heres some pics of the extractor fan, the soil pipe and another vent that leads to the roof.
Bathroom extractor fan:
http://imgur.com/MQxXlaZ,ZsczdGY,uao4vaS,n2c4tus,HsofcH0,826uwkH,N6NMZcH,C494wDO#0
Suspected soil pipe...