Speaking with my Building Surveyors (hard)hat on and being qualified to remove asbestos (although I don't choose to do so) it has been correctly stated by others that asbestos cement roofing sheets are NOT a licenced material. Therefore, as long as they are double bagged and disposed of at a licenced disposal site, anyone can do the work, it doesn't need to be a licenced contractor.
As has also been said, if it is in good condition, it is best left where it is, as long as it is subject to regular inspection to ensure condition does not deteriorate.
As to whos liability it is to remove, in my understanding the bottom line is that the 'duty holder' would ultimately be whoever is responsible for the up-keep and maintenance of the building, not necessarily the owner. If it is a privately owned shed, even if on a council site, then the 'owner' would be liable. Even if it were a council owned shed, if the allotment lease put responsibility on the plot holder to repair / maintain the shed, it would be the plot holder who would be liable.
I would have to qualify this (I am a surveyor after all!) by saying that my knowledge is based on non-domestic (i.e. commercial) buildings. The rules on domestic dwellings are differant, but I don't know how an allotment shed would be classed.
Richard