It's being eaten by caterpillars and aphids.

Use an insecticide called Provado and in quite a short space of time the plant will recover.
They're tough plants and once the infestation has gone the plant'll be fine.

A lot of my shrubs look just like yours and the biggest damage is those damn caterpillars - incidently, it would be worth spraying all the shrubs in the garden because they're likely to be on other ones too.
Spray, liberally, at the beginning of each season for the next year so that the caterpillars can be properly eradicated.

I've checked on the RHS site, and wondered if this article and advice might help ID the pest. Copy and paste it into your browser.

http://apps.rhs.org.uk/advicesearch/Profile.aspx?pid=556

Thank you!

Off to buy some provado. Is it possible to spray it too much?

Did wonder what the horrible smell was and its sounds like the rotting leaves from this plant.

I use provado on problem plants aprox four times a year and works well, i use this on viburnham tinus which does suffer from shothole [viburnham beetle]

I have 2 viburnums as part of a tall hedge. One of them is badly affected by shothole this year. I have never had it before and the plants are about 15-20 years old. The affected plant is recovering well but if it succumbs again next year, (as it probably will, I suppose, after this infestation) I will have it out and plant something else instead.
My garden is wildlife friendly so I don't use insecticides in it. I don't have much trouble from pests as a rule. My main enemy is the deadly lily beetle!