Thanks to everyone who gave me an idea what the buggy board was like on the other thread, it seems as though it's not a great idea.
Been having a look about online at doubles, and it's terrifying. Wheels falling off, too heavy to move, front-heavy with a big toddler.
The only one I can find that seems pretty well-loved is the Phil and Ted's Sport. I will try and get a second hand one, with the doubles kit already included etc.
Does anyone else have any recommendations? Largeish and lively 3yr old DS, upstairs flat which means it has to be easy to fold and compact when down. I also use buses a lot so it has to be easy to push and not monstrous.
Thanks again everyone for all your help ... this is my biggest purchase and I REALLY need to get it right as we don't have a lot of money at the moment and I don't want to be stuck with something I hate.