First ever post, so excuse me if you know this already, but GreenGear of the US has convertible tandem/triplet bikes that can fold down or come apart. There must be somewhere in the UK you can buy them. Must admit I've never tried one, but they look very nice. Possibly slightly more flex than a full size frame, but whether that is relevant depends on whether your goal is racing or puddling about with family. Good luck. Personally, I have a Dahon and it's changed my outlook on small wheel bikes completely. Now looking to go really strange and get myself a tandem recumbent somewhere.
