Are you sure its a 'bug' eating the plant? Where is it, in the ground or in a pot? What exactly are the symptoms you're seeing?

It's in the ground, south facing garden, next to a fence running along the southern boundary, (so adjacent north facing side of the fence), 4 leylandii the other side of the fence - although now just cut down, many, many dead brown leaves, just a few green shoots, no visible bugs, dont think it's lack of water, err ...that's about it.

I hope you're saying the Leylandii on the other side of the fence have been cut down, or even better, removed. If they're directly behind the border you're speaking about, your hydrangea will be suffering not only drought, but shortage of nutrients. Leylandii suck out all the moisture and pinch all the nutrients, so anything planted within 6 to 10 feet will be suffering. And that might just be the explanation as to why you keep losing them...

thank you, lets hope the removal of leylandii will mean this hydrangea can cling on to life!