Is this a "sick" tree?

It could be there are weeping forms of ash tree

Try Googling weeping ash tree leaf to see of you can confirm your tree is an ash

50 INCHES! That is a touch over 4 feet. Bit small for a mature tree.
A picture of the leaves etc. would be helpful in diagnosing the problem.

And a twig, I saw a photograph of a small branch black on one side

Do you get dwarf ash?
I agree a photo is needed to help with the id.

I am not aware of a dwarf Ash... we really need a photo or a clear i.d. from you Lucy

Yes., for some reason I missed the bit about it being mature at 50 inches

Some species of Mountain Ash (Sorbus) only grow to about four feet but this is a different genus to the common ash tree and I don't know if the disease affects them or not - I do hope not.
One problem is that, at this time of year, most trees look as if they could be dying as they drop their leaves.

They call it Chalaera fraxinea. or something like that, ash being Fraxinus excelsior as you say Bulba, and Rowan being Sorbus acuparia it should not affect them

We did have a young one die here when the ground was too wet where it was planted
Two nice ones going now though in adifferent area

There are a lot of ash around us here, they predominate in the hawthorn hedges. We had two huge ones bordering the road that had to be removed some years ago

It is only Fraxinus which is affected by chalara fraxinea - Sorbus (commonly known as Mountain Ash) is not.

Dreadful news though, it seems to be sweeping the country, places in Scotland affected now I understand

Still waiting for a response from the original poster.

Oh yes........

Which I suspect we are not going to get...

To be honest, I think it very unlikely your tree is an Ash of any kind - the commonest weeping trees are Kilmarnock Willow and Prunus 'kiku shidare', so the cause of the die back is likely to be some other infection or pest, and there are plenty of other things which may cause die back, some serious, some not.