More usefully, I now know how to put the chain back on. Apparently it's designed to be tight at the top and loose at the bottom. I couldn't put it back on because I was trying to pull the chain up from the top where it's pulled tight already. What you have to do is sort of push it up from the bottom so that you have more length of chain to work with. I guess that sort of thing is blindingly obvious to an engineer, but it wasn't to me. So I mention it solely for the education of others.
Speaking of education, I seem to have become some sort of tricycle missionary. Three times in the past two weeks, I've been stopped in the street by people wanting to know where I got the tricycle from, because they didn't previously know tricycles-for-adults existed and they want to get one. I'm thinking that I need to put together a flyer with all the companies' phone numbers and URLs on, that I can just give to people when they ask. It's very strange. Oh well!