Viburnum tinus

Help! I have had 2 mature Viburnum tinus in my garden which have suddenly died.The first one went last summer and the other is going in exactly the same way at the moment.They started with a small section looking rather like they're wilting but soon go brown and die off.The rest of the plant follows suit and it all happens quite qickley (a couple of months). They are in semi shade under an Oak tree,which also looks ill,in  clay soil.

Hi. I can't tell from the picture, but do the leaves have a lace-doily like appearance and is there a rotting smell especially after a shower?

Thanks for that but it has neither.I've done some more research and think it may be Phytophtora root rot?

The same thing has been happening to me for the last 5 years, and I have lost 4 viburnum tinus plants, with 2 more that have started dying very quickly within the last 2 weeks. Part of the plant takes on a leathery appearance, leaves turn yellow from the bottom of the stem upwards. Those that haven't fallen off then go brown and wither, and eventually the whole plant dies in the space of a month or two. No one can tell me what is wrong, although sudden oak death has been mentioned. Help - I have a garden full of these plants!!!