Q.My lovely plum tree is not leafing out!
The plum tree in my Mediterranean garden is not leafing out. For the past two seasons, this usually vigorous tree of eleven years has produced very little fruit and this spring, no leaves. There appear to be small buds and there was some blossoming. Have also found over last couple of years numbers of dead bees. Could this have been the reason or could rainfall levels or harsher winter this year have had something to do with it? Our hedge has some fungus issues too. Is this a factor or is my lovely, formerly fruitful, tree dying?

All of the issues you mention very well may be factors in the failure of your Plum tree to leaf out.
I have listed a few links for you.
https://www.gardeningknowhow.com/edible/fruits/plum/growing-plum-trees.htm
https://www.gardeningknowhow.com/edible/fruits/plum/plum-tree-diseases.htm
https://www.gardeningknowhow.com/garden-how-to/beneficial/insect-pollination-process.htm
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm