have a large Robinia False Acacia tree which has always florished and had plenty of leaves. This year it hardly has any with some branches being completely bare. I want to save this tree, I would be grateful for any advice on what the problem is and wha

Perhaps its been infested by something - you need to inspect the leaves higher up to see if you can find anything, mould, growths, sooty deposits, twisted leaves, insects, scale on the bark, check the trunk for signs of disease, splits, fungus, etc.

I have noticed the same problem on several robinia in my area. The RHS has also had a lot of queries on the same. The main suspect is fungal leaf spot. They are asking for samples and photos at https://www.rhs.org.uk/advice/profiles0708/robinia.asp

If all fails you can cut the tree down, allow suckers to form / grow. Select the strongest, cut out the rest and a new tree. I read this some years ago so you will need to research. It must only be as a last resort mind as there are no guarantees.