Small pidgeon in our garden in the pouring rain, what can we do to help
best advice is always to leave these things to nature as the adult birds may be returning when you are not watching and if it is still in the nest then it is safer there than on the ground. Chances are parent birds will return to the nest if they have young there (they are fed on food secretions called "crop milk" from both parents) so give it more time; I believe they leave the nest around 30 day after hatching; pigeons tend to stay in group flocks. So for now, keep a watch and distance from the nesting site so hopefully one or both of the adults can attend to their offspring. Good luck.
post edit: it may be a good idea to change your username (email) as this site is open to the world wide web and you may get spam if you keep your email as your username.