Hydrangea seems to have woodworm holes in the old wood

when you say 'old' wood, do you mean dead wood? if so, remove all dead wood - many hydrangeas become congested and dead wood should be removed anyway. Woodworm, so far as I know, only inhabits dead wood, and isn't a problem outdoors, though I can't swear to that. As you're in the UK, we're fortunate enough not to have 'borers' like they do in the States - post a photo if you're still not sure what to do.

Thanks Bamboo..........I think it's dead wood which needs to be removed as you say, it's very congested.