I would say if it's couch grass that can't be hand-weeded, take the shrub out if possible, clean the soil of all roots, then replant and water it well as if it were a new shrub. You can't use weedkiller, for obvious reasons.

Thanks for your idea. Much appreciated. Certainly worth a try.

You can use weed killer if you use it carefully. Use a small paint brush and carefully apply round up to some of the couch grass stems. As long as you don't get it on the cotoneaster it will be fine.

I've done this several times and it works well but is a bit fiddly.

I cut the bottom off a pop bottle, place over the grass and spray through the top, ensuring no part of the shrub is in the way of the spray.