Waste outlet pipe size

pcoe
pcoe Forum Participant Posts: 3
edited February 2014 in Caravans #1

I have a 1994 compass lynx 

I'm having trouble obtaining the correct size pipe to fit the waste outlet socket !

ive been told  early 90's caravans had smaller sockets

however no one seems to know what size pipe I need to fit! I've tried to measure roughly and it appears to be 20-21 mm the guy at my local camoping/caravan shop doesn't seem that interested in helping 

does this sound correct?  Also where would I obtain such pipr

many thanks in advance