Mountain ash issue

@Loofah Mountain ash Sorbus acucuparia is a member of Rosaceae andis more closely related to roses, apples etc. Ash Fraxinus is more closely related to the olive.
Good news Ash dieback won't affect your Mountain ash; bad news they are prone to fireblight, apple canker, silver leaf and honey fungus. Last years hot dry summer will have stressed it. if a smallish branch I would just remove it, yours seems to be one of the major branches, though, removal will significantly alter the tree, so maybe consider removing the tree.
What's the climber that can be seen in the pictures?

 

Just to say that ash dieback is specific to true ash (fraxinus) while 'mountain ash'/ rowan is actually a sorbus so wouldn't be affected. More about ash dieback

Hopefully someone else can help ID your tree and what ails it. What is the other plant twining round the trunk and some of the branches? Maybe that has contributed to the stress. It looks a find big specimen so it can be restored to health!

PS Oops I see NigelJ provided the help while I was writing this!

 

It looks a fairly old rowan tree, it might just be reaching its final days, slowly.
The climber possibly dead Ivy?

 

I had a smallish one in the front garden it went into a decline after about 30 years or so, with branches dying back and the bark splitting. So I took.it out one weekend and as much of the root as possible as it was producing a lot of suckers.

 

I dont think I've seen one that big TBH.
They mostly tend to be varieties grafted onto rootstocks.

To get that big I'd guess its a species tree and been there a long time.

I think its one of those kind of trees that has a limited life in the south of the country may live longer further north or in harder growing areas of the country.

 

It could probably go on for years just living with all the scars, or could die next year if it is just nearing the end of its life.
Bit like people really, someone looking really healthy drops dead while someone can look like they are at deaths door for years.