does anyone know for certain what the legal situation is....dont they need
permission from all the people who share/own the communal garden?

Forget the planning permission bits, any land that is owned (or has
license
to be occupied) requires the permission of the land owners (occupiers)
for
something to happen to it, just through natural law. If you are the
sole
owner (occupier), then implicitly automatically give yourself
permission to
do whatever you want with it, but if others are involved, all need to
be involved.
Imagine if you were half of a couple who owned a house and the other
half
decided - by themselves - to sell it. They can't, unless in some cases
they
own more than 50% of it.

In your case, there are more than one owner (occupier), so all persons
permission is needed. If shed-man hasn't got the permission of all the
people involved he is breaking some sort of law, maybe joint ownership
law of some sort. What would happen in the above case if hubby sold
a shared house without wifie's consent?