Mine is a Raleigh Adventurer. Very affordable.
However, a quick search reveals that Raleigh appear to have discontinued this model for 2013. Out of stock in most outlets, even Halfords don't have any (they did last year). There are a few mail order places that still have it in stock apparently, try a Googol 'shopping' search.
Alternatives include the similar, slightly cheaper, but not identical Viking tandems. All rigid MTB style with 26" wheels. These were mostly unavailable last year or I would have considered them. The Viking Serengeti is the nearest to the Raleigh, with disc brakes. Slightly different 'ladyback' alloy frame and hi-tensile forks (the Raleigh has CroMo forks), smaller disc rotors, but pretty similar spec otherwise. A fair bit cheaper too.
If V brakes are acceptable (could always add a rear disc brake as a 'drag' brake) then Viking have an even cheaper tandem, the Valhalla; and the near identical Timber Trail. They also have the steel gas pipe framed Timberwolf as their bargain basement option. I've seen one of these locked to railings in the Southside somewhere last year and it looked okay.
Viking also do/did a 700c wheeled 'touring' tandem complete with rear carrier and mudguards: the Aviemore. Not much use for a child stoker, but a useful option for two adults. Not sure how easily available it is though.
If you want something with a better reputation and higher quality, but under a grand the Dawes Duet is worth a look. Alloy frame, V brakes, rear carrier, mudguards. There's a childback version available too.