how do I drain my waste water tank, talbot express swift capri 1989?

We have just bought this van and we are unable to work out how to drain the waste water tank, there does not seem to be a drain tap anywhere under the van, there is a pipe which runs from the tank to just behind the side door. The pipe is open but seems to be permanently fixed with a clip. The instruction book simply says to drain the tank regularly and warns if overfull it can back up into the shower. Any ideas??