I would say that it has had its day. There is nothing to say that even though there is some re-gowth it won't happen again sooner than later. Why not grow another somewhere else in the garden? I would leave any planting in the same spot for a while. Perhaps some 'Rootgrow' when you finally re-plant.

Viburnum tinus does seem to suffer these symptoms. I have one whose branches died off one by one and I cut them off as they died and for now it looks OK. Another one in a completely different spot died more quickly of the same thing. I'm not having any more! I would replace with something different.

I concur with above. It's over. The shoots are only using up the reserve. Your viburnum did not agree with its site for some reason and finally gave up. Long periods of wet soggy weather would do it. Full sun & good drainage are 2 essentials. With that, very little else bothers them. I would encourage you to get another one of these wonderful shrubs. So many choices these days. This link will give you more detailed care for viburnum.

https://gardeningwithcharlie.com/grow-care-for-viburnum.html

we also had a very long hot dry summer, [for us] and that may also be a cause. try cutting the healthy shoot off with any roots you can find attached to it and pot it up. if it was happy for 25 yrs then the actual aspect was ok.

Thanks all. I think the consensus is definitely to give it up! I will try and salvage the live offshoot and pot it up, but I’ll give the spot a rest for a while and try something else later. Grateful for all advice and suggestions.

By the way - the site has good drainage and is in full sun for almost the entire day. The viburnum has always been a good doer - until now :)

i suspect the dry weather the most likely cause then. as a mature shrub there would have been a lot of leaf to loose water through.
welcome to Goy too: meant to say that yesterday but got distracted :o)