Established Magnolia tree - can it be saved?

[sadmagnolia]

We were lucky enough to move into a house with a lovely established magnolia in the front garden. It flowered and held it's leaves well last year but this year it's struggling. About 1/4 to 1/3 of the branches have not grown leaves after winter (the others flowered) and are  smaller branches are dry and crumbly to touch. Some have side branches with leaves and others without. I haven't spotted any particular pattern to it. We did some work in a flower bed nearby this spring and spread soil enricher from an Anaerobic Digestor around but otherwise have not gone near it. 

 

I am worried the tree is dying. What might be going on here, is there anything I can do to save it? It's such a lovely tree we would hate to lose it. Any help would be gratefully appreciated

Edited May 29 by sadmagnolia