I'm with Charlotte on this one. Cheap, old tandem for a pub bike - fine.
But the stresses on a tandem frame and components when riding more than a couple of mile pootle to the pub would mean I wouldn't want to risk trying this on something older than hadn't had a lot of money and attention spent on it - even when I see perfectly restored older tandems, they still terrify me!
You will often see good stuff around the £400 mark that will be a far safer purchase - as Charlotte said, you're less likely to break something, it should have working gears (and more importantly, BRAKES!).
I think a cheaper, older tandem would be a false economy. It wouldn't be enjoyable to ride very far and would probably end up becoming unloved and unused fairly quickly - whereas a half-decent modern tandem would last for years to come, wouldn't try and kill you on a regular basis etc.
Best of luck!