9793 Dying hazel tree?

I know there are other guys in another thread talking about hazel trees but thought this was a bit of a different subject so warranted a new query...I wasn't sure...anyway....the issue is this:

We've got a (probably old) hazelnut/cob tree in the garden and since we've been here it has always cropped very well. BUT this year MOST of the tree is looking very sickly - lots of small twigs are completely bare of leaves and the leaves which exist are small and fairly shrivelled compared to those on the rest of the healthier parts which remain. Do you think this is a temporary problem or is it on its way out.....?

There ARE small nuts forming all over the tree - but mainly on the healthier bits.

I suspect it is suffering from old age (but maybe it's a pest or disease) and would like to coax it to survive as it provides welcome shade to our patio.

The tree has 3 main 'trunks'. Most of the scraggy bit are on what looks to be the oldest 'trunk'.

If it's suffering from some kind of blight, can I do anything about it? If I leave it alone, does it sounds like something the tree might recover from? If it's just age, would it help if I cut out the old, dodgy-looking parts and (hopefully) kept the newer, healthier bits?Or is it most likely to be in terminal decline and on its way to the great woodyard in the sky (via the wood-burning stove).

I'd welcome any thoughts/ideas!