need to replace my cotoneaster prostratusCampanula

I have a rockery in my front garden 8m x 4m which has various shrubs in it. I planted cotoneaster prostratus in it about 20 years ago as ground cover. It certainly works well as far as covering the ground goes and I have to cut it back every now and again. My problem is that grass grows through it every year, sometimes I pull it out, sometimes I strim it. We have a grassy area across the road from the house so grass seeds blow over. I would like this part of the garden to be lower maintenance that it is at the present.
I'd like to replace it with another ground cover plant that won't allow the grass to come through, we have Campanula in various parts of the garden and they work well as ground cover, we also have Geraniums which do the same job.
Ideas please.

Marion