Hayley, I am terribly sorry to read this news but I am not surprised. I still think it is worth going to the media.
I don't want to be too personal...but are the ashes buried in an urn? Or just in the soil? I'm not a solicitor, but I think that if the ashes are kept separate and buried in their urns, then your case is much stronger.....if their ashes were mixed with the soil, then I believe it doesn't qualify as a "cemetery" iykwim--so the legal things pertaining to cemeteries wouldn't apply.
I am TOTALLY on your side....it shouldn't matter, because the point is that you placed their remains there on the assurance that it would be a permanent resting place. It's still a "sacred space" for you and for many other people. If you have the soil from the garden moved now, (you said to a new place on hospital grounds?) can they say that they will never build or tamper with THAT area?
HUGS. I'm furious on your behalf.