OK, I think we'd better call it solved!
I had his name as Natalio Horcajo Garcia but I've also seen it as Natalio Garcia Horcajo and just as plain old Natalio Horcaja too, so I've no idea what's right!
This is the one and only Mini-Break Practico from 1956 with an air-cooled twin designed by Horcajo himself.
Well done.