When you think about it all they seem to be is a go cart with a bigger chair on and handlebars , anyone got a blueprint
What do you recon the sort of electric motor it would need , a frame welded together a few cogs and body panels ..job done 
Easier & cheaper to take a standard mobility scoot, rip it back to a rolling chassis, and then build your own body for it I reckon.
The DfT do have rules that govern what defines an electrically assisted bicycle (mainly top speed & weight), and I wouldn't mind betting there is similar legislation that defines when a scoot becomes a motorised vehicle and therefore requires insurance, etc.
I reckon there could be a real market for something like the Landy that Symes has built, although I also reckon there is a real possibility of some a$$wipe eurocrat introducing laws to stop even the most basic of fun like that.
Having said that, those rules more than likely exist already