Re: Now that ElReg is...
Takes about two weeks to make one from kit. Friend did exactly that. Others can take much longer. Took me some months to make a Westfield 25 years ago, but that included sourcing a lot of parts myself. Caterham kits have most of the bits you need already.
My Locost still isn't finished and I started it over ten years ago. But then I did make the chassis and build the engine from bits. (ex. FFord engine, +60 overbore, twin dellorto 40's, stage 3 head, high lift cam etc).
Building these things is fun, unless you have children. When you never have time.